diff options
Diffstat (limited to 'Documentation/locking')
| -rw-r--r-- | Documentation/locking/futex-requeue-pi.rst | 132 | ||||
| -rw-r--r-- | Documentation/locking/hwspinlock.rst | 485 | ||||
| -rw-r--r-- | Documentation/locking/index.rst | 7 | ||||
| -rw-r--r-- | Documentation/locking/locktorture.rst | 2 | ||||
| -rw-r--r-- | Documentation/locking/percpu-rw-semaphore.rst | 28 | ||||
| -rw-r--r-- | Documentation/locking/pi-futex.rst | 122 | ||||
| -rw-r--r-- | Documentation/locking/preempt-locking.rst | 144 | ||||
| -rw-r--r-- | Documentation/locking/robust-futex-ABI.rst | 184 | ||||
| -rw-r--r-- | Documentation/locking/robust-futexes.rst | 221 | ||||
| -rw-r--r-- | Documentation/locking/rt-mutex.rst | 2 |
10 files changed, 1325 insertions, 2 deletions
diff --git a/Documentation/locking/futex-requeue-pi.rst b/Documentation/locking/futex-requeue-pi.rst new file mode 100644 index 000000000000..14ab5787b9a7 --- /dev/null +++ b/Documentation/locking/futex-requeue-pi.rst @@ -0,0 +1,132 @@ +================ +Futex Requeue PI +================ + +Requeueing of tasks from a non-PI futex to a PI futex requires +special handling in order to ensure the underlying rt_mutex is never +left without an owner if it has waiters; doing so would break the PI +boosting logic [see rt-mutex-desgin.txt] For the purposes of +brevity, this action will be referred to as "requeue_pi" throughout +this document. Priority inheritance is abbreviated throughout as +"PI". + +Motivation +---------- + +Without requeue_pi, the glibc implementation of +pthread_cond_broadcast() must resort to waking all the tasks waiting +on a pthread_condvar and letting them try to sort out which task +gets to run first in classic thundering-herd formation. An ideal +implementation would wake the highest-priority waiter, and leave the +rest to the natural wakeup inherent in unlocking the mutex +associated with the condvar. + +Consider the simplified glibc calls:: + + /* caller must lock mutex */ + pthread_cond_wait(cond, mutex) + { + lock(cond->__data.__lock); + unlock(mutex); + do { + unlock(cond->__data.__lock); + futex_wait(cond->__data.__futex); + lock(cond->__data.__lock); + } while(...) + unlock(cond->__data.__lock); + lock(mutex); + } + + pthread_cond_broadcast(cond) + { + lock(cond->__data.__lock); + unlock(cond->__data.__lock); + futex_requeue(cond->data.__futex, cond->mutex); + } + +Once pthread_cond_broadcast() requeues the tasks, the cond->mutex +has waiters. Note that pthread_cond_wait() attempts to lock the +mutex only after it has returned to user space. This will leave the +underlying rt_mutex with waiters, and no owner, breaking the +previously mentioned PI-boosting algorithms. + +In order to support PI-aware pthread_condvar's, the kernel needs to +be able to requeue tasks to PI futexes. This support implies that +upon a successful futex_wait system call, the caller would return to +user space already holding the PI futex. The glibc implementation +would be modified as follows:: + + + /* caller must lock mutex */ + pthread_cond_wait_pi(cond, mutex) + { + lock(cond->__data.__lock); + unlock(mutex); + do { + unlock(cond->__data.__lock); + futex_wait_requeue_pi(cond->__data.__futex); + lock(cond->__data.__lock); + } while(...) + unlock(cond->__data.__lock); + /* the kernel acquired the mutex for us */ + } + + pthread_cond_broadcast_pi(cond) + { + lock(cond->__data.__lock); + unlock(cond->__data.__lock); + futex_requeue_pi(cond->data.__futex, cond->mutex); + } + +The actual glibc implementation will likely test for PI and make the +necessary changes inside the existing calls rather than creating new +calls for the PI cases. Similar changes are needed for +pthread_cond_timedwait() and pthread_cond_signal(). + +Implementation +-------------- + +In order to ensure the rt_mutex has an owner if it has waiters, it +is necessary for both the requeue code, as well as the waiting code, +to be able to acquire the rt_mutex before returning to user space. +The requeue code cannot simply wake the waiter and leave it to +acquire the rt_mutex as it would open a race window between the +requeue call returning to user space and the waiter waking and +starting to run. This is especially true in the uncontended case. + +The solution involves two new rt_mutex helper routines, +rt_mutex_start_proxy_lock() and rt_mutex_finish_proxy_lock(), which +allow the requeue code to acquire an uncontended rt_mutex on behalf +of the waiter and to enqueue the waiter on a contended rt_mutex. +Two new system calls provide the kernel<->user interface to +requeue_pi: FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI. + +FUTEX_WAIT_REQUEUE_PI is called by the waiter (pthread_cond_wait() +and pthread_cond_timedwait()) to block on the initial futex and wait +to be requeued to a PI-aware futex. The implementation is the +result of a high-speed collision between futex_wait() and +futex_lock_pi(), with some extra logic to check for the additional +wake-up scenarios. + +FUTEX_CMP_REQUEUE_PI is called by the waker +(pthread_cond_broadcast() and pthread_cond_signal()) to requeue and +possibly wake the waiting tasks. Internally, this system call is +still handled by futex_requeue (by passing requeue_pi=1). Before +requeueing, futex_requeue() attempts to acquire the requeue target +PI futex on behalf of the top waiter. If it can, this waiter is +woken. futex_requeue() then proceeds to requeue the remaining +nr_wake+nr_requeue tasks to the PI futex, calling +rt_mutex_start_proxy_lock() prior to each requeue to prepare the +task as a waiter on the underlying rt_mutex. It is possible that +the lock can be acquired at this stage as well, if so, the next +waiter is woken to finish the acquisition of the lock. + +FUTEX_CMP_REQUEUE_PI accepts nr_wake and nr_requeue as arguments, but +their sum is all that really matters. futex_requeue() will wake or +requeue up to nr_wake + nr_requeue tasks. It will wake only as many +tasks as it can acquire the lock for, which in the majority of cases +should be 0 as good programming practice dictates that the caller of +either pthread_cond_broadcast() or pthread_cond_signal() acquire the +mutex prior to making the call. FUTEX_CMP_REQUEUE_PI requires that +nr_wake=1. nr_requeue should be INT_MAX for broadcast and 0 for +signal. diff --git a/Documentation/locking/hwspinlock.rst b/Documentation/locking/hwspinlock.rst new file mode 100644 index 000000000000..6f03713b7003 --- /dev/null +++ b/Documentation/locking/hwspinlock.rst @@ -0,0 +1,485 @@ +=========================== +Hardware Spinlock Framework +=========================== + +Introduction +============ + +Hardware spinlock modules provide hardware assistance for synchronization +and mutual exclusion between heterogeneous processors and those not operating +under a single, shared operating system. + +For example, OMAP4 has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP, +each of which is running a different Operating System (the master, A9, +is usually running Linux and the slave processors, the M3 and the DSP, +are running some flavor of RTOS). + +A generic hwspinlock framework allows platform-independent drivers to use +the hwspinlock device in order to access data structures that are shared +between remote processors, that otherwise have no alternative mechanism +to accomplish synchronization and mutual exclusion operations. + +This is necessary, for example, for Inter-processor communications: +on OMAP4, cpu-intensive multimedia tasks are offloaded by the host to the +remote M3 and/or C64x+ slave processors (by an IPC subsystem called Syslink). + +To achieve fast message-based communications, a minimal kernel support +is needed to deliver messages arriving from a remote processor to the +appropriate user process. + +This communication is based on simple data structures that is shared between +the remote processors, and access to it is synchronized using the hwspinlock +module (remote processor directly places new messages in this shared data +structure). + +A common hwspinlock interface makes it possible to have generic, platform- +independent, drivers. + +User API +======== + +:: + + struct hwspinlock *hwspin_lock_request(void); + +Dynamically assign an hwspinlock and return its address, or NULL +in case an unused hwspinlock isn't available. Users of this +API will usually want to communicate the lock's id to the remote core +before it can be used to achieve synchronization. + +Should be called from a process context (might sleep). + +:: + + struct hwspinlock *hwspin_lock_request_specific(unsigned int id); + +Assign a specific hwspinlock id and return its address, or NULL +if that hwspinlock is already in use. Usually board code will +be calling this function in order to reserve specific hwspinlock +ids for predefined purposes. + +Should be called from a process context (might sleep). + +:: + + int of_hwspin_lock_get_id(struct device_node *np, int index); + +Retrieve the global lock id for an OF phandle-based specific lock. +This function provides a means for DT users of a hwspinlock module +to get the global lock id of a specific hwspinlock, so that it can +be requested using the normal hwspin_lock_request_specific() API. + +The function returns a lock id number on success, -EPROBE_DEFER if +the hwspinlock device is not yet registered with the core, or other +error values. + +Should be called from a process context (might sleep). + +:: + + int hwspin_lock_free(struct hwspinlock *hwlock); + +Free a previously-assigned hwspinlock; returns 0 on success, or an +appropriate error code on failure (e.g. -EINVAL if the hwspinlock +is already free). + +Should be called from a process context (might sleep). + +:: + + int hwspin_lock_timeout(struct hwspinlock *hwlock, unsigned int timeout); + +Lock a previously-assigned hwspinlock with a timeout limit (specified in +msecs). If the hwspinlock is already taken, the function will busy loop +waiting for it to be released, but give up when the timeout elapses. +Upon a successful return from this function, preemption is disabled so +the caller must not sleep, and is advised to release the hwspinlock as +soon as possible, in order to minimize remote cores polling on the +hardware interconnect. + +Returns 0 when successful and an appropriate error code otherwise (most +notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs). +The function will never sleep. + +:: + + int hwspin_lock_timeout_irq(struct hwspinlock *hwlock, unsigned int timeout); + +Lock a previously-assigned hwspinlock with a timeout limit (specified in +msecs). If the hwspinlock is already taken, the function will busy loop +waiting for it to be released, but give up when the timeout elapses. +Upon a successful return from this function, preemption and the local +interrupts are disabled, so the caller must not sleep, and is advised to +release the hwspinlock as soon as possible. + +Returns 0 when successful and an appropriate error code otherwise (most +notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs). +The function will never sleep. + +:: + + int hwspin_lock_timeout_irqsave(struct hwspinlock *hwlock, unsigned int to, + unsigned long *flags); + +Lock a previously-assigned hwspinlock with a timeout limit (specified in +msecs). If the hwspinlock is already taken, the function will busy loop +waiting for it to be released, but give up when the timeout elapses. +Upon a successful return from this function, preemption is disabled, +local interrupts are disabled and their previous state is saved at the +given flags placeholder. The caller must not sleep, and is advised to +release the hwspinlock as soon as possible. + +Returns 0 when successful and an appropriate error code otherwise (most +notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs). + +The function will never sleep. + +:: + + int hwspin_lock_timeout_raw(struct hwspinlock *hwlock, unsigned int timeout); + +Lock a previously-assigned hwspinlock with a timeout limit (specified in +msecs). If the hwspinlock is already taken, the function will busy loop +waiting for it to be released, but give up when the timeout elapses. + +Caution: User must protect the routine of getting hardware lock with mutex +or spinlock to avoid dead-lock, that will let user can do some time-consuming +or sleepable operations under the hardware lock. + +Returns 0 when successful and an appropriate error code otherwise (most +notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs). + +The function will never sleep. + +:: + + int hwspin_lock_timeout_in_atomic(struct hwspinlock *hwlock, unsigned int to); + +Lock a previously-assigned hwspinlock with a timeout limit (specified in +msecs). If the hwspinlock is already taken, the function will busy loop +waiting for it to be released, but give up when the timeout elapses. + +This function shall be called only from an atomic context and the timeout +value shall not exceed a few msecs. + +Returns 0 when successful and an appropriate error code otherwise (most +notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs). + +The function will never sleep. + +:: + + int hwspin_trylock(struct hwspinlock *hwlock); + + +Attempt to lock a previously-assigned hwspinlock, but immediately fail if +it is already taken. + +Upon a successful return from this function, preemption is disabled so +caller must not sleep, and is advised to release the hwspinlock as soon as +possible, in order to minimize remote cores polling on the hardware +interconnect. + +Returns 0 on success and an appropriate error code otherwise (most +notably -EBUSY if the hwspinlock was already taken). +The function will never sleep. + +:: + + int hwspin_trylock_irq(struct hwspinlock *hwlock); + + +Attempt to lock a previously-assigned hwspinlock, but immediately fail if +it is already taken. + +Upon a successful return from this function, preemption and the local +interrupts are disabled so caller must not sleep, and is advised to +release the hwspinlock as soon as possible. + +Returns 0 on success and an appropriate error code otherwise (most +notably -EBUSY if the hwspinlock was already taken). + +The function will never sleep. + +:: + + int hwspin_trylock_irqsave(struct hwspinlock *hwlock, unsigned long *flags); + +Attempt to lock a previously-assigned hwspinlock, but immediately fail if +it is already taken. + +Upon a successful return from this function, preemption is disabled, +the local interrupts are disabled and their previous state is saved +at the given flags placeholder. The caller must not sleep, and is advised +to release the hwspinlock as soon as possible. + +Returns 0 on success and an appropriate error code otherwise (most +notably -EBUSY if the hwspinlock was already taken). +The function will never sleep. + +:: + + int hwspin_trylock_raw(struct hwspinlock *hwlock); + +Attempt to lock a previously-assigned hwspinlock, but immediately fail if +it is already taken. + +Caution: User must protect the routine of getting hardware lock with mutex +or spinlock to avoid dead-lock, that will let user can do some time-consuming +or sleepable operations under the hardware lock. + +Returns 0 on success and an appropriate error code otherwise (most +notably -EBUSY if the hwspinlock was already taken). +The function will never sleep. + +:: + + int hwspin_trylock_in_atomic(struct hwspinlock *hwlock); + +Attempt to lock a previously-assigned hwspinlock, but immediately fail if +it is already taken. + +This function shall be called only from an atomic context. + +Returns 0 on success and an appropriate error code otherwise (most +notably -EBUSY if the hwspinlock was already taken). +The function will never sleep. + +:: + + void hwspin_unlock(struct hwspinlock *hwlock); + +Unlock a previously-locked hwspinlock. Always succeed, and can be called +from any context (the function never sleeps). + +.. note:: + + code should **never** unlock an hwspinlock which is already unlocked + (there is no protection against this). + +:: + + void hwspin_unlock_irq(struct hwspinlock *hwlock); + +Unlock a previously-locked hwspinlock and enable local interrupts. +The caller should **never** unlock an hwspinlock which is already unlocked. + +Doing so is considered a bug (there is no protection against this). +Upon a successful return from this function, preemption and local +interrupts are enabled. This function will never sleep. + +:: + + void + hwspin_unlock_irqrestore(struct hwspinlock *hwlock, unsigned long *flags); + +Unlock a previously-locked hwspinlock. + +The caller should **never** unlock an hwspinlock which is already unlocked. +Doing so is considered a bug (there is no protection against this). +Upon a successful return from this function, preemption is reenabled, +and the state of the local interrupts is restored to the state saved at +the given flags. This function will never sleep. + +:: + + void hwspin_unlock_raw(struct hwspinlock *hwlock); + +Unlock a previously-locked hwspinlock. + +The caller should **never** unlock an hwspinlock which is already unlocked. +Doing so is considered a bug (there is no protection against this). +This function will never sleep. + +:: + + void hwspin_unlock_in_atomic(struct hwspinlock *hwlock); + +Unlock a previously-locked hwspinlock. + +The caller should **never** unlock an hwspinlock which is already unlocked. +Doing so is considered a bug (there is no protection against this). +This function will never sleep. + +:: + + int hwspin_lock_get_id(struct hwspinlock *hwlock); + +Retrieve id number of a given hwspinlock. This is needed when an +hwspinlock is dynamically assigned: before it can be used to achieve +mutual exclusion with a remote cpu, the id number should be communicated +to the remote task with which we want to synchronize. + +Returns the hwspinlock id number, or -EINVAL if hwlock is null. + +Typical usage +============= + +:: + + #include <linux/hwspinlock.h> + #include <linux/err.h> + + int hwspinlock_example1(void) + { + struct hwspinlock *hwlock; + int ret; + + /* dynamically assign a hwspinlock */ + hwlock = hwspin_lock_request(); + if (!hwlock) + ... + + id = hwspin_lock_get_id(hwlock); + /* probably need to communicate id to a remote processor now */ + + /* take the lock, spin for 1 sec if it's already taken */ + ret = hwspin_lock_timeout(hwlock, 1000); + if (ret) + ... + + /* + * we took the lock, do our thing now, but do NOT sleep + */ + + /* release the lock */ + hwspin_unlock(hwlock); + + /* free the lock */ + ret = hwspin_lock_free(hwlock); + if (ret) + ... + + return ret; + } + + int hwspinlock_example2(void) + { + struct hwspinlock *hwlock; + int ret; + + /* + * assign a specific hwspinlock id - this should be called early + * by board init code. + */ + hwlock = hwspin_lock_request_specific(PREDEFINED_LOCK_ID); + if (!hwlock) + ... + + /* try to take it, but don't spin on it */ + ret = hwspin_trylock(hwlock); + if (!ret) { + pr_info("lock is already taken\n"); + return -EBUSY; + } + + /* + * we took the lock, do our thing now, but do NOT sleep + */ + + /* release the lock */ + hwspin_unlock(hwlock); + + /* free the lock */ + ret = hwspin_lock_free(hwlock); + if (ret) + ... + + return ret; + } + + +API for implementors +==================== + +:: + + int hwspin_lock_register(struct hwspinlock_device *bank, struct device *dev, + const struct hwspinlock_ops *ops, int base_id, int num_locks); + +To be called from the underlying platform-specific implementation, in +order to register a new hwspinlock device (which is usually a bank of +numerous locks). Should be called from a process context (this function +might sleep). + +Returns 0 on success, or appropriate error code on failure. + +:: + + int hwspin_lock_unregister(struct hwspinlock_device *bank); + +To be called from the underlying vendor-specific implementation, in order +to unregister an hwspinlock device (which is usually a bank of numerous +locks). + +Should be called from a process context (this function might sleep). + +Returns the address of hwspinlock on success, or NULL on error (e.g. +if the hwspinlock is still in use). + +Important structs +================= + +struct hwspinlock_device is a device which usually contains a bank +of hardware locks. It is registered by the underlying hwspinlock +implementation using the hwspin_lock_register() API. + +:: + + /** + * struct hwspinlock_device - a device which usually spans numerous hwspinlocks + * @dev: underlying device, will be used to invoke runtime PM api + * @ops: platform-specific hwspinlock handlers + * @base_id: id index of the first lock in this device + * @num_locks: number of locks in this device + * @lock: dynamically allocated array of 'struct hwspinlock' + */ + struct hwspinlock_device { + struct device *dev; + const struct hwspinlock_ops *ops; + int base_id; + int num_locks; + struct hwspinlock lock[0]; + }; + +struct hwspinlock_device contains an array of hwspinlock structs, each +of which represents a single hardware lock:: + + /** + * struct hwspinlock - this struct represents a single hwspinlock instance + * @bank: the hwspinlock_device structure which owns this lock + * @lock: initialized and used by hwspinlock core + * @priv: private data, owned by the underlying platform-specific hwspinlock drv + */ + struct hwspinlock { + struct hwspinlock_device *bank; + spinlock_t lock; + void *priv; + }; + +When registering a bank of locks, the hwspinlock driver only needs to +set the priv members of the locks. The rest of the members are set and +initialized by the hwspinlock core itself. + +Implementation callbacks +======================== + +There are three possible callbacks defined in 'struct hwspinlock_ops':: + + struct hwspinlock_ops { + int (*trylock)(struct hwspinlock *lock); + void (*unlock)(struct hwspinlock *lock); + void (*relax)(struct hwspinlock *lock); + }; + +The first two callbacks are mandatory: + +The ->trylock() callback should make a single attempt to take the lock, and +return 0 on failure and 1 on success. This callback may **not** sleep. + +The ->unlock() callback releases the lock. It always succeed, and it, too, +may **not** sleep. + +The ->relax() callback is optional. It is called by hwspinlock core while +spinning on a lock, and can be used by the underlying implementation to force +a delay between two successive invocations of ->trylock(). It may **not** sleep. diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst index 5d6800a723dc..d785878cad65 100644 --- a/Documentation/locking/index.rst +++ b/Documentation/locking/index.rst @@ -16,6 +16,13 @@ locking rt-mutex spinlocks ww-mutex-design + preempt-locking + pi-futex + futex-requeue-pi + hwspinlock + percpu-rw-semaphore + robust-futexes + robust-futex-ABI .. only:: subproject and html diff --git a/Documentation/locking/locktorture.rst b/Documentation/locking/locktorture.rst index 5bcb99ba7bd9..8012a74555e7 100644 --- a/Documentation/locking/locktorture.rst +++ b/Documentation/locking/locktorture.rst @@ -110,7 +110,7 @@ stutter same period of time. Defaults to "stutter=5", so as to run and pause for (roughly) five-second intervals. Specifying "stutter=0" causes the test to run continuously - without pausing, which is the old default behavior. + without pausing. shuffle_interval The number of seconds to keep the test threads affinitied diff --git a/Documentation/locking/percpu-rw-semaphore.rst b/Documentation/locking/percpu-rw-semaphore.rst new file mode 100644 index 000000000000..247de6410855 --- /dev/null +++ b/Documentation/locking/percpu-rw-semaphore.rst @@ -0,0 +1,28 @@ +==================== +Percpu rw semaphores +==================== + +Percpu rw semaphores is a new read-write semaphore design that is +optimized for locking for reading. + +The problem with traditional read-write semaphores is that when multiple +cores take the lock for reading, the cache line containing the semaphore +is bouncing between L1 caches of the cores, causing performance +degradation. + +Locking for reading is very fast, it uses RCU and it avoids any atomic +instruction in the lock and unlock path. On the other hand, locking for +writing is very expensive, it calls synchronize_rcu() that can take +hundreds of milliseconds. + +The lock is declared with "struct percpu_rw_semaphore" type. +The lock is initialized percpu_init_rwsem, it returns 0 on success and +-ENOMEM on allocation failure. +The lock must be freed with percpu_free_rwsem to avoid memory leak. + +The lock is locked for read with percpu_down_read, percpu_up_read and +for write with percpu_down_write, percpu_up_write. + +The idea of using RCU for optimized rw-lock was introduced by +Eric Dumazet <eric.dumazet@gmail.com>. +The code was written by Mikulas Patocka <mpatocka@redhat.com> diff --git a/Documentation/locking/pi-futex.rst b/Documentation/locking/pi-futex.rst new file mode 100644 index 000000000000..c33ba2befbf8 --- /dev/null +++ b/Documentation/locking/pi-futex.rst @@ -0,0 +1,122 @@ +====================== +Lightweight PI-futexes +====================== + +We are calling them lightweight for 3 reasons: + + - in the user-space fastpath a PI-enabled futex involves no kernel work + (or any other PI complexity) at all. No registration, no extra kernel + calls - just pure fast atomic ops in userspace. + + - even in the slowpath, the system call and scheduling pattern is very + similar to normal futexes. + + - the in-kernel PI implementation is streamlined around the mutex + abstraction, with strict rules that keep the implementation + relatively simple: only a single owner may own a lock (i.e. no + read-write lock support), only the owner may unlock a lock, no + recursive locking, etc. + +Priority Inheritance - why? +--------------------------- + +The short reply: user-space PI helps achieving/improving determinism for +user-space applications. In the best-case, it can help achieve +determinism and well-bound latencies. Even in the worst-case, PI will +improve the statistical distribution of locking related application +delays. + +The longer reply +---------------- + +Firstly, sharing locks between multiple tasks is a common programming +technique that often cannot be replaced with lockless algorithms. As we +can see it in the kernel [which is a quite complex program in itself], +lockless structures are rather the exception than the norm - the current +ratio of lockless vs. locky code for shared data structures is somewhere +between 1:10 and 1:100. Lockless is hard, and the complexity of lockless +algorithms often endangers to ability to do robust reviews of said code. +I.e. critical RT apps often choose lock structures to protect critical +data structures, instead of lockless algorithms. Furthermore, there are +cases (like shared hardware, or other resource limits) where lockless +access is mathematically impossible. + +Media players (such as Jack) are an example of reasonable application +design with multiple tasks (with multiple priority levels) sharing +short-held locks: for example, a highprio audio playback thread is +combined with medium-prio construct-audio-data threads and low-prio +display-colory-stuff threads. Add video and decoding to the mix and +we've got even more priority levels. + +So once we accept that synchronization objects (locks) are an +unavoidable fact of life, and once we accept that multi-task userspace +apps have a very fair expectation of being able to use locks, we've got +to think about how to offer the option of a deterministic locking +implementation to user-space. + +Most of the technical counter-arguments against doing priority +inheritance only apply to kernel-space locks. But user-space locks are +different, there we cannot disable interrupts or make the task +non-preemptible in a critical section, so the 'use spinlocks' argument +does not apply (user-space spinlocks have the same priority inversion +problems as other user-space locking constructs). Fact is, pretty much +the only technique that currently enables good determinism for userspace +locks (such as futex-based pthread mutexes) is priority inheritance: + +Currently (without PI), if a high-prio and a low-prio task shares a lock +[this is a quite common scenario for most non-trivial RT applications], +even if all critical sections are coded carefully to be deterministic +(i.e. all critical sections are short in duration and only execute a +limited number of instructions), the kernel cannot guarantee any +deterministic execution of the high-prio task: any medium-priority task +could preempt the low-prio task while it holds the shared lock and +executes the critical section, and could delay it indefinitely. + +Implementation +-------------- + +As mentioned before, the userspace fastpath of PI-enabled pthread +mutexes involves no kernel work at all - they behave quite similarly to +normal futex-based locks: a 0 value means unlocked, and a value==TID +means locked. (This is the same method as used by list-based robust +futexes.) Userspace uses atomic ops to lock/unlock these mutexes without +entering the kernel. + +To handle the slowpath, we have added two new futex ops: + + - FUTEX_LOCK_PI + - FUTEX_UNLOCK_PI + +If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to +TID fails], then FUTEX_LOCK_PI is called. The kernel does all the +remaining work: if there is no futex-queue attached to the futex address +yet then the code looks up the task that owns the futex [it has put its +own TID into the futex value], and attaches a 'PI state' structure to +the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, +kernel-based synchronization object. The 'other' task is made the owner +of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the +futex value. Then this task tries to lock the rt-mutex, on which it +blocks. Once it returns, it has the mutex acquired, and it sets the +futex value to its own TID and returns. Userspace has no other work to +perform - it now owns the lock, and futex value contains +FUTEX_WAITERS|TID. + +If the unlock side fastpath succeeds, [i.e. userspace manages to do a +TID -> 0 atomic transition of the futex value], then no kernel work is +triggered. + +If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), +then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the +behalf of userspace - and it also unlocks the attached +pi_state->rt_mutex and thus wakes up any potential waiters. + +Note that under this approach, contrary to previous PI-futex approaches, +there is no prior 'registration' of a PI-futex. [which is not quite +possible anyway, due to existing ABI properties of pthread mutexes.] + +Also, under this scheme, 'robustness' and 'PI' are two orthogonal +properties of futexes, and all four combinations are possible: futex, +robust-futex, PI-futex, robust+PI-futex. + +More details about priority inheritance can be found in +Documentation/locking/rt-mutex.rst. diff --git a/Documentation/locking/preempt-locking.rst b/Documentation/locking/preempt-locking.rst new file mode 100644 index 000000000000..dce336134e54 --- /dev/null +++ b/Documentation/locking/preempt-locking.rst @@ -0,0 +1,144 @@ +=========================================================================== +Proper Locking Under a Preemptible Kernel: Keeping Kernel Code Preempt-Safe +=========================================================================== + +:Author: Robert Love <rml@tech9.net> + + +Introduction +============ + + +A preemptible kernel creates new locking issues. The issues are the same as +those under SMP: concurrency and reentrancy. Thankfully, the Linux preemptible +kernel model leverages existing SMP locking mechanisms. Thus, the kernel +requires explicit additional locking for very few additional situations. + +This document is for all kernel hackers. Developing code in the kernel +requires protecting these situations. + + +RULE #1: Per-CPU data structures need explicit protection +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + + +Two similar problems arise. An example code snippet:: + + struct this_needs_locking tux[NR_CPUS]; + tux[smp_processor_id()] = some_value; + /* task is preempted here... */ + something = tux[smp_processor_id()]; + +First, since the data is per-CPU, it may not have explicit SMP locking, but +require it otherwise. Second, when a preempted task is finally rescheduled, +the previous value of smp_processor_id may not equal the current. You must +protect these situations by disabling preemption around them. + +You can also use put_cpu() and get_cpu(), which will disable preemption. + + +RULE #2: CPU state must be protected. +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + + +Under preemption, the state of the CPU must be protected. This is arch- +dependent, but includes CPU structures and state not preserved over a context +switch. For example, on x86, entering and exiting FPU mode is now a critical +section that must occur while preemption is disabled. Think what would happen +if the kernel is executing a floating-point instruction and is then preempted. +Remember, the kernel does not save FPU state except for user tasks. Therefore, +upon preemption, the FPU registers will be sold to the lowest bidder. Thus, +preemption must be disabled around such regions. + +Note, some FPU functions are already explicitly preempt safe. For example, +kernel_fpu_begin and kernel_fpu_end will disable and enable preemption. + + +RULE #3: Lock acquire and release must be performed by same task +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + + +A lock acquired in one task must be released by the same task. This +means you can't do oddball things like acquire a lock and go off to +play while another task releases it. If you want to do something +like this, acquire and release the task in the same code path and +have the caller wait on an event by the other task. + + +Solution +======== + + +Data protection under preemption is achieved by disabling preemption for the +duration of the critical region. + +:: + + preempt_enable() decrement the preempt counter + preempt_disable() increment the preempt counter + preempt_enable_no_resched() decrement, but do not immediately preempt + preempt_check_resched() if needed, reschedule + preempt_count() return the preempt counter + +The functions are nestable. In other words, you can call preempt_disable +n-times in a code path, and preemption will not be reenabled until the n-th +call to preempt_enable. The preempt statements define to nothing if +preemption is not enabled. + +Note that you do not need to explicitly prevent preemption if you are holding +any locks or interrupts are disabled, since preemption is implicitly disabled +in those cases. + +But keep in mind that 'irqs disabled' is a fundamentally unsafe way of +disabling preemption - any cond_resched() or cond_resched_lock() might trigger +a reschedule if the preempt count is 0. A simple printk() might trigger a +reschedule. So use this implicit preemption-disabling property only if you +know that the affected codepath does not do any of this. Best policy is to use +this only for small, atomic code that you wrote and which calls no complex +functions. + +Example:: + + cpucache_t *cc; /* this is per-CPU */ + preempt_disable(); + cc = cc_data(searchp); + if (cc && cc->avail) { + __free_block(searchp, cc_entry(cc), cc->avail); + cc->avail = 0; + } + preempt_enable(); + return 0; + +Notice how the preemption statements must encompass every reference of the +critical variables. Another example:: + + int buf[NR_CPUS]; + set_cpu_val(buf); + if (buf[smp_processor_id()] == -1) printf(KERN_INFO "wee!\n"); + spin_lock(&buf_lock); + /* ... */ + +This code is not preempt-safe, but see how easily we can fix it by simply +moving the spin_lock up two lines. + + +Preventing preemption using interrupt disabling +=============================================== + + +It is possible to prevent a preemption event using local_irq_disable and +local_irq_save. Note, when doing so, you must be very careful to not cause +an event that would set need_resched and result in a preemption check. When +in doubt, rely on locking or explicit preemption disabling. + +Note in 2.5 interrupt disabling is now only per-CPU (e.g. local). + +An additional concern is proper usage of local_irq_disable and local_irq_save. +These may be used to protect from preemption, however, on exit, if preemption +may be enabled, a test to see if preemption is required should be done. If +these are called from the spin_lock and read/write lock macros, the right thing +is done. They may also be called within a spin-lock protected region, however, +if they are ever called outside of this context, a test for preemption should +be made. Do note that calls from interrupt context or bottom half/ tasklets +are also protected by preemption locks and so may use the versions which do +not check preemption. diff --git a/Documentation/locking/robust-futex-ABI.rst b/Documentation/locking/robust-futex-ABI.rst new file mode 100644 index 000000000000..f24904f1c16f --- /dev/null +++ b/Documentation/locking/robust-futex-ABI.rst @@ -0,0 +1,184 @@ +==================== +The robust futex ABI +==================== + +:Author: Started by Paul Jackson <pj@sgi.com> + + +Robust_futexes provide a mechanism that is used in addition to normal +futexes, for kernel assist of cleanup of held locks on task exit. + +The interesting data as to what futexes a thread is holding is kept on a +linked list in user space, where it can be updated efficiently as locks +are taken and dropped, without kernel intervention. The only additional +kernel intervention required for robust_futexes above and beyond what is +required for futexes is: + + 1) a one time call, per thread, to tell the kernel where its list of + held robust_futexes begins, and + 2) internal kernel code at exit, to handle any listed locks held + by the exiting thread. + +The existing normal futexes already provide a "Fast Userspace Locking" +mechanism, which handles uncontested locking without needing a system +call, and handles contested locking by maintaining a list of waiting +threads in the kernel. Options on the sys_futex(2) system call support +waiting on a particular futex, and waking up the next waiter on a +particular futex. + +For robust_futexes to work, the user code (typically in a library such +as glibc linked with the application) has to manage and place the +necessary list elements exactly as the kernel expects them. If it fails +to do so, then improperly listed locks will not be cleaned up on exit, +probably causing deadlock or other such failure of the other threads +waiting on the same locks. + +A thread that anticipates possibly using robust_futexes should first +issue the system call:: + + asmlinkage long + sys_set_robust_list(struct robust_list_head __user *head, size_t len); + +The pointer 'head' points to a structure in the threads address space +consisting of three words. Each word is 32 bits on 32 bit arch's, or 64 +bits on 64 bit arch's, and local byte order. Each thread should have +its own thread private 'head'. + +If a thread is running in 32 bit compatibility mode on a 64 native arch +kernel, then it can actually have two such structures - one using 32 bit +words for 32 bit compatibility mode, and one using 64 bit words for 64 +bit native mode. The kernel, if it is a 64 bit kernel supporting 32 bit +compatibility mode, will attempt to process both lists on each task +exit, if the corresponding sys_set_robust_list() call has been made to +setup that list. + + The first word in the memory structure at 'head' contains a + pointer to a single linked list of 'lock entries', one per lock, + as described below. If the list is empty, the pointer will point + to itself, 'head'. The last 'lock entry' points back to the 'head'. + + The second word, called 'offset', specifies the offset from the + address of the associated 'lock entry', plus or minus, of what will + be called the 'lock word', from that 'lock entry'. The 'lock word' + is always a 32 bit word, unlike the other words above. The 'lock + word' holds 2 flag bits in the upper 2 bits, and the thread id (TID) + of the thread holding the lock in the bottom 30 bits. See further + below for a description of the flag bits. + + The third word, called 'list_op_pending', contains transient copy of + the address of the 'lock entry', during list insertion and removal, + and is needed to correctly resolve races should a thread exit while + in the middle of a locking or unlocking operation. + +Each 'lock entry' on the single linked list starting at 'head' consists +of just a single word, pointing to the next 'lock entry', or back to +'head' if there are no more entries. In addition, nearby to each 'lock +entry', at an offset from the 'lock entry' specified by the 'offset' +word, is one 'lock word'. + +The 'lock word' is always 32 bits, and is intended to be the same 32 bit +lock variable used by the futex mechanism, in conjunction with +robust_futexes. The kernel will only be able to wakeup the next thread +waiting for a lock on a threads exit if that next thread used the futex +mechanism to register the address of that 'lock word' with the kernel. + +For each futex lock currently held by a thread, if it wants this +robust_futex support for exit cleanup of that lock, it should have one +'lock entry' on this list, with its associated 'lock word' at the +specified 'offset'. Should a thread die while holding any such locks, +the kernel will walk this list, mark any such locks with a bit +indicating their holder died, and wakeup the next thread waiting for +that lock using the futex mechanism. + +When a thread has invoked the above system call to indicate it +anticipates using robust_futexes, the kernel stores the passed in 'head' +pointer for that task. The task may retrieve that value later on by +using the system call:: + + asmlinkage long + sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr, + size_t __user *len_ptr); + +It is anticipated that threads will use robust_futexes embedded in +larger, user level locking structures, one per lock. The kernel +robust_futex mechanism doesn't care what else is in that structure, so +long as the 'offset' to the 'lock word' is the same for all +robust_futexes used by that thread. The thread should link those locks +it currently holds using the 'lock entry' pointers. It may also have +other links between the locks, such as the reverse side of a double +linked list, but that doesn't matter to the kernel. + +By keeping its locks linked this way, on a list starting with a 'head' +pointer known to the kernel, the kernel can provide to a thread the +essential service available for robust_futexes, which is to help clean +up locks held at the time of (a perhaps unexpectedly) exit. + +Actual locking and unlocking, during normal operations, is handled +entirely by user level code in the contending threads, and by the +existing futex mechanism to wait for, and wakeup, locks. The kernels +only essential involvement in robust_futexes is to remember where the +list 'head' is, and to walk the list on thread exit, handling locks +still held by the departing thread, as described below. + +There may exist thousands of futex lock structures in a threads shared +memory, on various data structures, at a given point in time. Only those +lock structures for locks currently held by that thread should be on +that thread's robust_futex linked lock list a given time. + +A given futex lock structure in a user shared memory region may be held +at different times by any of the threads with access to that region. The +thread currently holding such a lock, if any, is marked with the threads +TID in the lower 30 bits of the 'lock word'. + +When adding or removing a lock from its list of held locks, in order for +the kernel to correctly handle lock cleanup regardless of when the task +exits (perhaps it gets an unexpected signal 9 in the middle of +manipulating this list), the user code must observe the following +protocol on 'lock entry' insertion and removal: + +On insertion: + + 1) set the 'list_op_pending' word to the address of the 'lock entry' + to be inserted, + 2) acquire the futex lock, + 3) add the lock entry, with its thread id (TID) in the bottom 30 bits + of the 'lock word', to the linked list starting at 'head', and + 4) clear the 'list_op_pending' word. + +On removal: + + 1) set the 'list_op_pending' word to the address of the 'lock entry' + to be removed, + 2) remove the lock entry for this lock from the 'head' list, + 3) release the futex lock, and + 4) clear the 'lock_op_pending' word. + +On exit, the kernel will consider the address stored in +'list_op_pending' and the address of each 'lock word' found by walking +the list starting at 'head'. For each such address, if the bottom 30 +bits of the 'lock word' at offset 'offset' from that address equals the +exiting threads TID, then the kernel will do two things: + + 1) if bit 31 (0x80000000) is set in that word, then attempt a futex + wakeup on that address, which will waken the next thread that has + used to the futex mechanism to wait on that address, and + 2) atomically set bit 30 (0x40000000) in the 'lock word'. + +In the above, bit 31 was set by futex waiters on that lock to indicate +they were waiting, and bit 30 is set by the kernel to indicate that the +lock owner died holding the lock. + +The kernel exit code will silently stop scanning the list further if at +any point: + + 1) the 'head' pointer or an subsequent linked list pointer + is not a valid address of a user space word + 2) the calculated location of the 'lock word' (address plus + 'offset') is not the valid address of a 32 bit user space + word + 3) if the list contains more than 1 million (subject to + future kernel configuration changes) elements. + +When the kernel sees a list entry whose 'lock word' doesn't have the +current threads TID in the lower 30 bits, it does nothing with that +entry, and goes on to the next entry. diff --git a/Documentation/locking/robust-futexes.rst b/Documentation/locking/robust-futexes.rst new file mode 100644 index 000000000000..6361fb01c9c1 --- /dev/null +++ b/Documentation/locking/robust-futexes.rst @@ -0,0 +1,221 @@ +======================================== +A description of what robust futexes are +======================================== + +:Started by: Ingo Molnar <mingo@redhat.com> + +Background +---------- + +what are robust futexes? To answer that, we first need to understand +what futexes are: normal futexes are special types of locks that in the +noncontended case can be acquired/released from userspace without having +to enter the kernel. + +A futex is in essence a user-space address, e.g. a 32-bit lock variable +field. If userspace notices contention (the lock is already owned and +someone else wants to grab it too) then the lock is marked with a value +that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT) +syscall is used to wait for the other guy to release it. The kernel +creates a 'futex queue' internally, so that it can later on match up the +waiter with the waker - without them having to know about each other. +When the owner thread releases the futex, it notices (via the variable +value) that there were waiter(s) pending, and does the +sys_futex(FUTEX_WAKE) syscall to wake them up. Once all waiters have +taken and released the lock, the futex is again back to 'uncontended' +state, and there's no in-kernel state associated with it. The kernel +completely forgets that there ever was a futex at that address. This +method makes futexes very lightweight and scalable. + +"Robustness" is about dealing with crashes while holding a lock: if a +process exits prematurely while holding a pthread_mutex_t lock that is +also shared with some other process (e.g. yum segfaults while holding a +pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need +to be notified that the last owner of the lock exited in some irregular +way. + +To solve such types of problems, "robust mutex" userspace APIs were +created: pthread_mutex_lock() returns an error value if the owner exits +prematurely - and the new owner can decide whether the data protected by +the lock can be recovered safely. + +There is a big conceptual problem with futex based mutexes though: it is +the kernel that destroys the owner task (e.g. due to a SEGFAULT), but +the kernel cannot help with the cleanup: if there is no 'futex queue' +(and in most cases there is none, futexes being fast lightweight locks) +then the kernel has no information to clean up after the held lock! +Userspace has no chance to clean up after the lock either - userspace is +the one that crashes, so it has no opportunity to clean up. Catch-22. + +In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot +is needed to release that futex based lock. This is one of the leading +bugreports against yum. + +To solve this problem, the traditional approach was to extend the vma +(virtual memory area descriptor) concept to have a notion of 'pending +robust futexes attached to this area'. This approach requires 3 new +syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and +FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether +they have a robust_head set. This approach has two fundamental problems +left: + + - it has quite complex locking and race scenarios. The vma-based + approach had been pending for years, but they are still not completely + reliable. + + - they have to scan _every_ vma at sys_exit() time, per thread! + +The second disadvantage is a real killer: pthread_exit() takes around 1 +microsecond on Linux, but with thousands (or tens of thousands) of vmas +every pthread_exit() takes a millisecond or more, also totally +destroying the CPU's L1 and L2 caches! + +This is very much noticeable even for normal process sys_exit_group() +calls: the kernel has to do the vma scanning unconditionally! (this is +because the kernel has no knowledge about how many robust futexes there +are to be cleaned up, because a robust futex might have been registered +in another task, and the futex variable might have been simply mmap()-ed +into this process's address space). + +This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that +normal kernels can turn it off, but worse than that: the overhead makes +robust futexes impractical for any type of generic Linux distribution. + +So something had to be done. + +New approach to robust futexes +------------------------------ + +At the heart of this new approach there is a per-thread private list of +robust locks that userspace is holding (maintained by glibc) - which +userspace list is registered with the kernel via a new syscall [this +registration happens at most once per thread lifetime]. At do_exit() +time, the kernel checks this user-space list: are there any robust futex +locks to be cleaned up? + +In the common case, at do_exit() time, there is no list registered, so +the cost of robust futexes is just a simple current->robust_list != NULL +comparison. If the thread has registered a list, then normally the list +is empty. If the thread/process crashed or terminated in some incorrect +way then the list might be non-empty: in this case the kernel carefully +walks the list [not trusting it], and marks all locks that are owned by +this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if +any). + +The list is guaranteed to be private and per-thread at do_exit() time, +so it can be accessed by the kernel in a lockless way. + +There is one race possible though: since adding to and removing from the +list is done after the futex is acquired by glibc, there is a few +instructions window for the thread (or process) to die there, leaving +the futex hung. To protect against this possibility, userspace (glibc) +also maintains a simple per-thread 'list_op_pending' field, to allow the +kernel to clean up if the thread dies after acquiring the lock, but just +before it could have added itself to the list. Glibc sets this +list_op_pending field before it tries to acquire the futex, and clears +it after the list-add (or list-remove) has finished. + +That's all that is needed - all the rest of robust-futex cleanup is done +in userspace [just like with the previous patches]. + +Ulrich Drepper has implemented the necessary glibc support for this new +mechanism, which fully enables robust mutexes. + +Key differences of this userspace-list based approach, compared to the +vma based method: + + - it's much, much faster: at thread exit time, there's no need to loop + over every vma (!), which the VM-based method has to do. Only a very + simple 'is the list empty' op is done. + + - no VM changes are needed - 'struct address_space' is left alone. + + - no registration of individual locks is needed: robust mutexes don't + need any extra per-lock syscalls. Robust mutexes thus become a very + lightweight primitive - so they don't force the application designer + to do a hard choice between performance and robustness - robust + mutexes are just as fast. + + - no per-lock kernel allocation happens. + + - no resource limits are needed. + + - no kernel-space recovery call (FUTEX_RECOVER) is needed. + + - the implementation and the locking is "obvious", and there are no + interactions with the VM. + +Performance +----------- + +I have benchmarked the time needed for the kernel to process a list of 1 +million (!) held locks, using the new method [on a 2GHz CPU]: + + - with FUTEX_WAIT set [contended mutex]: 130 msecs + - without FUTEX_WAIT set [uncontended mutex]: 30 msecs + +I have also measured an approach where glibc does the lock notification +[which it currently does for !pshared robust mutexes], and that took 256 +msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls +userspace had to do. + +(1 million held locks are unheard of - we expect at most a handful of +locks to be held at a time. Nevertheless it's nice to know that this +approach scales nicely.) + +Implementation details +---------------------- + +The patch adds two new syscalls: one to register the userspace list, and +one to query the registered list pointer:: + + asmlinkage long + sys_set_robust_list(struct robust_list_head __user *head, + size_t len); + + asmlinkage long + sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr, + size_t __user *len_ptr); + +List registration is very fast: the pointer is simply stored in +current->robust_list. [Note that in the future, if robust futexes become +widespread, we could extend sys_clone() to register a robust-list head +for new threads, without the need of another syscall.] + +So there is virtually zero overhead for tasks not using robust futexes, +and even for robust futex users, there is only one extra syscall per +thread lifetime, and the cleanup operation, if it happens, is fast and +straightforward. The kernel doesn't have any internal distinction between +robust and normal futexes. + +If a futex is found to be held at exit time, the kernel sets the +following bit of the futex word:: + + #define FUTEX_OWNER_DIED 0x40000000 + +and wakes up the next futex waiter (if any). User-space does the rest of +the cleanup. + +Otherwise, robust futexes are acquired by glibc by putting the TID into +the futex field atomically. Waiters set the FUTEX_WAITERS bit:: + + #define FUTEX_WAITERS 0x80000000 + +and the remaining bits are for the TID. + +Testing, architecture support +----------------------------- + +I've tested the new syscalls on x86 and x86_64, and have made sure the +parsing of the userspace list is robust [ ;-) ] even if the list is +deliberately corrupted. + +i386 and x86_64 syscalls are wired up at the moment, and Ulrich has +tested the new glibc code (on x86_64 and i386), and it works for his +robust-mutex testcases. + +All other architectures should build just fine too - but they won't have +the new syscalls yet. + +Architectures need to implement the new futex_atomic_cmpxchg_inatomic() +inline function before writing up the syscalls. diff --git a/Documentation/locking/rt-mutex.rst b/Documentation/locking/rt-mutex.rst index c365dc302081..3b5097a380e6 100644 --- a/Documentation/locking/rt-mutex.rst +++ b/Documentation/locking/rt-mutex.rst @@ -4,7 +4,7 @@ RT-mutex subsystem with PI support RT-mutexes with priority inheritance are used to support PI-futexes, which enable pthread_mutex_t priority inheritance attributes -(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details +(PTHREAD_PRIO_INHERIT). [See Documentation/locking/pi-futex.rst for more details about PI-futexes.] This technology was developed in the -rt tree and streamlined for |