From: Sebastian Andrzej Siewior Date: Mon, 24 Oct 2022 10:17:22 +0200 Subject: [PATCH 345/353] timers: Don't block on ->expiry_lock for TIMER_IRQSAFE timers Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=e0550b85976f3d13ebffc2c98b3601d28c24b651 Upstream commit c725dafc95f1b37027840aaeaa8b7e4e9cd20516 PREEMPT_RT does not spin and wait until a running timer completes its callback but instead it blocks on a sleeping lock to prevent a livelock in the case that the task waiting for the callback completion preempted the callback. This cannot be done for timers flagged with TIMER_IRQSAFE. These timers can be canceled from an interrupt disabled context even on RT kernels. The expiry callback of such timers is invoked with interrupts disabled so there is no need to use the expiry lock mechanism because obviously the callback cannot be preempted even on RT kernels. Do not use the timer_base::expiry_lock mechanism when waiting for a running callback to complete if the timer is flagged with TIMER_IRQSAFE. Also add a lockdep assertion for RT kernels to validate that the expiry lock mechanism is always invoked in preemptible context. Reported-by: Mike Galbraith Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/20201103190937.hga67rqhvknki3tp@linutronix.de [bigeasy: The logic in v4.19 is slightly different but the outcome is the same as we must not sleep while waiting for the irqsafe timer to complete. The IRQSAFE timer can not be preempted. The "lockdep annotation" is not available and has been replaced with might_sleep()] Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Daniel Wagner --- kernel/time/timer.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 3e2c0bd03004..0a6d60b3e67c 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -1272,6 +1272,9 @@ static int __del_timer_sync(struct timer_list *timer) if (ret >= 0) return ret; + if (READ_ONCE(timer->flags) & TIMER_IRQSAFE) + continue; + /* * When accessing the lock, timers of base are no longer expired * and so timer is no longer running. @@ -1336,6 +1339,12 @@ int del_timer_sync(struct timer_list *timer) * could lead to deadlock. */ WARN_ON(in_irq() && !(timer->flags & TIMER_IRQSAFE)); + /* + * Must be able to sleep on PREEMPT_RT because of the slowpath in + * __del_timer_sync(). + */ + if (IS_ENABLED(CONFIG_PREEMPT_RT) && !(timer->flags & TIMER_IRQSAFE)) + might_sleep(); return __del_timer_sync(timer); }