summaryrefslogtreecommitdiffstats
path: root/kernel/rcu/srcu.c
diff options
context:
space:
mode:
authorLance Roy <ldr709@gmail.com>2017-01-23 22:35:18 +0100
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-01-25 21:53:20 +0100
commitf2c4689640e9a34bc45c013032185ed4ce47e7ff (patch)
treeb83abdbccba941253a9296379e00d09ce355ec2d /kernel/rcu/srcu.c
parentrcu: Narrow early boot window of illegal synchronous grace periods (diff)
downloadlinux-f2c4689640e9a34bc45c013032185ed4ce47e7ff.tar.xz
linux-f2c4689640e9a34bc45c013032185ed4ce47e7ff.zip
srcu: Implement more-efficient reader counts
SRCU uses two per-cpu counters: a nesting counter to count the number of active critical sections, and a sequence counter to ensure that the nesting counters don't change while they are being added together in srcu_readers_active_idx_check(). This patch instead uses per-cpu lock and unlock counters. Because both counters only increase and srcu_readers_active_idx_check() reads the unlock counter before the lock counter, this achieves the same end without having to increment two different counters in srcu_read_lock(). This also saves a smp_mb() in srcu_readers_active_idx_check(). Possible bug: There is no guarantee that the lock counter won't overflow during srcu_readers_active_idx_check(), as there are no memory barriers around srcu_flip() (see comment in srcu_readers_active_idx_check() for details). However, this problem was already present before this patch. Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Lance Roy <ldr709@gmail.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Lai Jiangshan <jiangshanlai@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/rcu/srcu.c')
-rw-r--r--kernel/rcu/srcu.c122
1 files changed, 44 insertions, 78 deletions
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index 9b9cdd549caa..c9a0015e1c2e 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -106,7 +106,7 @@ static int init_srcu_struct_fields(struct srcu_struct *sp)
rcu_batch_init(&sp->batch_check1);
rcu_batch_init(&sp->batch_done);
INIT_DELAYED_WORK(&sp->work, process_srcu);
- sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
+ sp->per_cpu_ref = alloc_percpu(struct srcu_array);
return sp->per_cpu_ref ? 0 : -ENOMEM;
}
@@ -141,114 +141,77 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
/*
- * Returns approximate total of the readers' ->seq[] values for the
+ * Returns approximate total of the readers' ->lock_count[] values for the
* rank of per-CPU counters specified by idx.
*/
-static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
+static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
{
int cpu;
unsigned long sum = 0;
- unsigned long t;
for_each_possible_cpu(cpu) {
- t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
- sum += t;
+ struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
+
+ sum += READ_ONCE(cpuc->lock_count[idx]);
}
return sum;
}
/*
- * Returns approximate number of readers active on the specified rank
- * of the per-CPU ->c[] counters.
+ * Returns approximate total of the readers' ->unlock_count[] values for the
+ * rank of per-CPU counters specified by idx.
*/
-static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
+static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
{
int cpu;
unsigned long sum = 0;
- unsigned long t;
for_each_possible_cpu(cpu) {
- t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
- sum += t;
+ struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
+
+ sum += READ_ONCE(cpuc->unlock_count[idx]);
}
return sum;
}
/*
* Return true if the number of pre-existing readers is determined to
- * be stably zero. An example unstable zero can occur if the call
- * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
- * but due to task migration, sees the corresponding __srcu_read_unlock()
- * decrement. This can happen because srcu_readers_active_idx() takes
- * time to sum the array, and might in fact be interrupted or preempted
- * partway through the summation.
+ * be zero.
*/
static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
{
- unsigned long seq;
+ unsigned long unlocks;
- seq = srcu_readers_seq_idx(sp, idx);
+ unlocks = srcu_readers_unlock_idx(sp, idx);
/*
- * The following smp_mb() A pairs with the smp_mb() B located in
- * __srcu_read_lock(). This pairing ensures that if an
- * __srcu_read_lock() increments its counter after the summation
- * in srcu_readers_active_idx(), then the corresponding SRCU read-side
- * critical section will see any changes made prior to the start
- * of the current SRCU grace period.
+ * Make sure that a lock is always counted if the corresponding unlock
+ * is counted. Needs to be a smp_mb() as the read side may contain a
+ * read from a variable that is written to before the synchronize_srcu()
+ * in the write side. In this case smp_mb()s A and B act like the store
+ * buffering pattern.
*
- * Also, if the above call to srcu_readers_seq_idx() saw the
- * increment of ->seq[], then the call to srcu_readers_active_idx()
- * must see the increment of ->c[].
+ * This smp_mb() also pairs with smp_mb() C to prevent accesses after the
+ * synchronize_srcu() from being executed before the grace period ends.
*/
smp_mb(); /* A */
/*
- * Note that srcu_readers_active_idx() can incorrectly return
- * zero even though there is a pre-existing reader throughout.
- * To see this, suppose that task A is in a very long SRCU
- * read-side critical section that started on CPU 0, and that
- * no other reader exists, so that the sum of the counters
- * is equal to one. Then suppose that task B starts executing
- * srcu_readers_active_idx(), summing up to CPU 1, and then that
- * task C starts reading on CPU 0, so that its increment is not
- * summed, but finishes reading on CPU 2, so that its decrement
- * -is- summed. Then when task B completes its sum, it will
- * incorrectly get zero, despite the fact that task A has been
- * in its SRCU read-side critical section the whole time.
+ * If the locks are the same as the unlocks, then there must have
+ * been no readers on this index at some time in between. This does not
+ * mean that there are no more readers, as one could have read the
+ * current index but not have incremented the lock counter yet.
*
- * We therefore do a validation step should srcu_readers_active_idx()
- * return zero.
+ * Possible bug: There is no guarantee that there haven't been ULONG_MAX
+ * increments of ->lock_count[] since the unlocks were counted, meaning
+ * that this could return true even if there are still active readers.
+ * Since there are no memory barriers around srcu_flip(), the CPU is not
+ * required to increment ->completed before running
+ * srcu_readers_unlock_idx(), which means that there could be an
+ * arbitrarily large number of critical sections that execute after
+ * srcu_readers_unlock_idx() but use the old value of ->completed.
*/
- if (srcu_readers_active_idx(sp, idx) != 0)
- return false;
-
- /*
- * The remainder of this function is the validation step.
- * The following smp_mb() D pairs with the smp_mb() C in
- * __srcu_read_unlock(). If the __srcu_read_unlock() was seen
- * by srcu_readers_active_idx() above, then any destructive
- * operation performed after the grace period will happen after
- * the corresponding SRCU read-side critical section.
- *
- * Note that there can be at most NR_CPUS worth of readers using
- * the old index, which is not enough to overflow even a 32-bit
- * integer. (Yes, this does mean that systems having more than
- * a billion or so CPUs need to be 64-bit systems.) Therefore,
- * the sum of the ->seq[] counters cannot possibly overflow.
- * Therefore, the only way that the return values of the two
- * calls to srcu_readers_seq_idx() can be equal is if there were
- * no increments of the corresponding rank of ->seq[] counts
- * in the interim. But the missed-increment scenario laid out
- * above includes an increment of the ->seq[] counter by
- * the corresponding __srcu_read_lock(). Therefore, if this
- * scenario occurs, the return values from the two calls to
- * srcu_readers_seq_idx() will differ, and thus the validation
- * step below suffices.
- */
- smp_mb(); /* D */
-
- return srcu_readers_seq_idx(sp, idx) == seq;
+ return srcu_readers_lock_idx(sp, idx) == unlocks;
}
/**
@@ -266,8 +229,12 @@ static bool srcu_readers_active(struct srcu_struct *sp)
unsigned long sum = 0;
for_each_possible_cpu(cpu) {
- sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
- sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
+ struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
+
+ sum += READ_ONCE(cpuc->lock_count[0]);
+ sum += READ_ONCE(cpuc->lock_count[1]);
+ sum -= READ_ONCE(cpuc->unlock_count[0]);
+ sum -= READ_ONCE(cpuc->unlock_count[1]);
}
return sum;
}
@@ -298,9 +265,8 @@ int __srcu_read_lock(struct srcu_struct *sp)
int idx;
idx = READ_ONCE(sp->completed) & 0x1;
- __this_cpu_inc(sp->per_cpu_ref->c[idx]);
+ __this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
smp_mb(); /* B */ /* Avoid leaking the critical section. */
- __this_cpu_inc(sp->per_cpu_ref->seq[idx]);
return idx;
}
EXPORT_SYMBOL_GPL(__srcu_read_lock);
@@ -314,7 +280,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
void __srcu_read_unlock(struct srcu_struct *sp, int idx)
{
smp_mb(); /* C */ /* Avoid leaking the critical section. */
- this_cpu_dec(sp->per_cpu_ref->c[idx]);
+ this_cpu_inc(sp->per_cpu_ref->unlock_count[idx]);
}
EXPORT_SYMBOL_GPL(__srcu_read_unlock);
@@ -349,7 +315,7 @@ static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
/*
* Increment the ->completed counter so that future SRCU readers will
- * use the other rank of the ->c[] and ->seq[] arrays. This allows
+ * use the other rank of the ->(un)lock_count[] arrays. This allows
* us to wait for pre-existing readers in a starvation-free manner.
*/
static void srcu_flip(struct srcu_struct *sp)