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authorPetr Mladek <pmladek@suse.com>2018-01-22 10:40:59 +0100
committerPetr Mladek <pmladek@suse.com>2018-01-22 10:40:59 +0100
commitbb4f552a598d40624343bc0815e594efd4d6ec3d (patch)
tree0c31c275252dfe7860110e980a28c438999acd3b /kernel/printk/printk.c
parentMerge branch 'for-4.16-print-symbol' into for-4.16 (diff)
parentprintk: Never set console_may_schedule in console_trylock() (diff)
downloadlinux-bb4f552a598d40624343bc0815e594efd4d6ec3d.tar.xz
linux-bb4f552a598d40624343bc0815e594efd4d6ec3d.zip
Merge branch 'for-4.16-console-waiter-logic' into for-4.16
Diffstat (limited to 'kernel/printk/printk.c')
-rw-r--r--kernel/printk/printk.c181
1 files changed, 166 insertions, 15 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index bf2e6741ec12..63416bb2712a 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1548,6 +1548,146 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
}
/*
+ * Special console_lock variants that help to reduce the risk of soft-lockups.
+ * They allow to pass console_lock to another printk() call using a busy wait.
+ */
+
+#ifdef CONFIG_LOCKDEP
+static struct lockdep_map console_owner_dep_map = {
+ .name = "console_owner"
+};
+#endif
+
+static DEFINE_RAW_SPINLOCK(console_owner_lock);
+static struct task_struct *console_owner;
+static bool console_waiter;
+
+/**
+ * console_lock_spinning_enable - mark beginning of code where another
+ * thread might safely busy wait
+ *
+ * This basically converts console_lock into a spinlock. This marks
+ * the section where the console_lock owner can not sleep, because
+ * there may be a waiter spinning (like a spinlock). Also it must be
+ * ready to hand over the lock at the end of the section.
+ */
+static void console_lock_spinning_enable(void)
+{
+ raw_spin_lock(&console_owner_lock);
+ console_owner = current;
+ raw_spin_unlock(&console_owner_lock);
+
+ /* The waiter may spin on us after setting console_owner */
+ spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+}
+
+/**
+ * console_lock_spinning_disable_and_check - mark end of code where another
+ * thread was able to busy wait and check if there is a waiter
+ *
+ * This is called at the end of the section where spinning is allowed.
+ * It has two functions. First, it is a signal that it is no longer
+ * safe to start busy waiting for the lock. Second, it checks if
+ * there is a busy waiter and passes the lock rights to her.
+ *
+ * Important: Callers lose the lock if there was a busy waiter.
+ * They must not touch items synchronized by console_lock
+ * in this case.
+ *
+ * Return: 1 if the lock rights were passed, 0 otherwise.
+ */
+static int console_lock_spinning_disable_and_check(void)
+{
+ int waiter;
+
+ raw_spin_lock(&console_owner_lock);
+ waiter = READ_ONCE(console_waiter);
+ console_owner = NULL;
+ raw_spin_unlock(&console_owner_lock);
+
+ if (!waiter) {
+ spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+ return 0;
+ }
+
+ /* The waiter is now free to continue */
+ WRITE_ONCE(console_waiter, false);
+
+ spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+
+ /*
+ * Hand off console_lock to waiter. The waiter will perform
+ * the up(). After this, the waiter is the console_lock owner.
+ */
+ mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
+ return 1;
+}
+
+/**
+ * console_trylock_spinning - try to get console_lock by busy waiting
+ *
+ * This allows to busy wait for the console_lock when the current
+ * owner is running in specially marked sections. It means that
+ * the current owner is running and cannot reschedule until it
+ * is ready to lose the lock.
+ *
+ * Return: 1 if we got the lock, 0 othrewise
+ */
+static int console_trylock_spinning(void)
+{
+ struct task_struct *owner = NULL;
+ bool waiter;
+ bool spin = false;
+ unsigned long flags;
+
+ if (console_trylock())
+ return 1;
+
+ printk_safe_enter_irqsave(flags);
+
+ raw_spin_lock(&console_owner_lock);
+ owner = READ_ONCE(console_owner);
+ waiter = READ_ONCE(console_waiter);
+ if (!waiter && owner && owner != current) {
+ WRITE_ONCE(console_waiter, true);
+ spin = true;
+ }
+ raw_spin_unlock(&console_owner_lock);
+
+ /*
+ * If there is an active printk() writing to the
+ * consoles, instead of having it write our data too,
+ * see if we can offload that load from the active
+ * printer, and do some printing ourselves.
+ * Go into a spin only if there isn't already a waiter
+ * spinning, and there is an active printer, and
+ * that active printer isn't us (recursive printk?).
+ */
+ if (!spin) {
+ printk_safe_exit_irqrestore(flags);
+ return 0;
+ }
+
+ /* We spin waiting for the owner to release us */
+ spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+ /* Owner will clear console_waiter on hand off */
+ while (READ_ONCE(console_waiter))
+ cpu_relax();
+ spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+
+ printk_safe_exit_irqrestore(flags);
+ /*
+ * The owner passed the console lock to us.
+ * Since we did not spin on console lock, annotate
+ * this as a trylock. Otherwise lockdep will
+ * complain.
+ */
+ mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
+
+ return 1;
+}
+
+/*
* Call the console drivers, asking them to write out
* log_buf[start] to log_buf[end - 1].
* The console_lock must be held.
@@ -1753,12 +1893,19 @@ asmlinkage int vprintk_emit(int facility, int level,
/* If called from the scheduler, we can not call up(). */
if (!in_sched) {
/*
+ * Disable preemption to avoid being preempted while holding
+ * console_sem which would prevent anyone from printing to
+ * console
+ */
+ preempt_disable();
+ /*
* Try to acquire and then immediately release the console
* semaphore. The release will print out buffers and wake up
* /dev/kmsg and syslog() users.
*/
- if (console_trylock())
+ if (console_trylock_spinning())
console_unlock();
+ preempt_enable();
}
return printed_len;
@@ -1859,6 +2006,8 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
static ssize_t msg_print_ext_body(char *buf, size_t size,
char *dict, size_t dict_len,
char *text, size_t text_len) { return 0; }
+static void console_lock_spinning_enable(void) { }
+static int console_lock_spinning_disable_and_check(void) { return 0; }
static void call_console_drivers(const char *ext_text, size_t ext_len,
const char *text, size_t len) {}
static size_t msg_print_text(const struct printk_log *msg,
@@ -2084,20 +2233,7 @@ int console_trylock(void)
return 0;
}
console_locked = 1;
- /*
- * When PREEMPT_COUNT disabled we can't reliably detect if it's
- * safe to schedule (e.g. calling printk while holding a spin_lock),
- * because preempt_disable()/preempt_enable() are just barriers there
- * and preempt_count() is always 0.
- *
- * RCU read sections have a separate preemption counter when
- * PREEMPT_RCU enabled thus we must take extra care and check
- * rcu_preempt_depth(), otherwise RCU read sections modify
- * preempt_count().
- */
- console_may_schedule = !oops_in_progress &&
- preemptible() &&
- !rcu_preempt_depth();
+ console_may_schedule = 0;
return 1;
}
EXPORT_SYMBOL(console_trylock);
@@ -2247,14 +2383,29 @@ skip:
console_seq++;
raw_spin_unlock(&logbuf_lock);
+ /*
+ * While actively printing out messages, if another printk()
+ * were to occur on another CPU, it may wait for this one to
+ * finish. This task can not be preempted if there is a
+ * waiter waiting to take over.
+ */
+ console_lock_spinning_enable();
+
stop_critical_timings(); /* don't trace print latency */
call_console_drivers(ext_text, ext_len, text, len);
start_critical_timings();
+
+ if (console_lock_spinning_disable_and_check()) {
+ printk_safe_exit_irqrestore(flags);
+ return;
+ }
+
printk_safe_exit_irqrestore(flags);
if (do_cond_resched)
cond_resched();
}
+
console_locked = 0;
/* Release the exclusive_console once it is used */