Messages in this thread Patch in this message | | | Date | Tue, 3 Nov 2020 20:09:37 +0100 | From | Sebastian Andrzej Siewior <> | Subject | [PATCH] timers: Don't block on ->expiry_lock for TIMER_IRQSAFE |
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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 deadlock.
This blocking can not be done for workqueue's IRQ_SAFE timer which will be canceled in an IRQ-off region. It has to happen to in IRQ-off region because changing the PENDING bit and clearing the timer must not be interrupted to avoid a busy-loop.
The callback invocation of IRQSAFE timer is not preempted on PREEMPT_RT so there is no need to synchronize on timer_base::expiry_lock.
Don't acquire the timer_base::expiry_lock for TIMER_IRQSAFE flagged timer. Add a lockdep annotation to ensure that this function is always invoked in preemptible context on PREEMPT_RT.
Reported-by: Mike Galbraith <efault@gmx.de> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- kernel/time/timer.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 7edc9fba34bbd..1e2403126ccfe 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -1288,7 +1288,7 @@ static void del_timer_wait_running(struct timer_list *timer) u32 tf; tf = READ_ONCE(timer->flags); - if (!(tf & TIMER_MIGRATING)) { + if (!(tf & (TIMER_MIGRATING | TIMER_IRQSAFE))) { struct timer_base *base = get_timer_base(tf); /* @@ -1372,6 +1372,13 @@ int del_timer_sync(struct timer_list *timer) */ WARN_ON(in_irq() && !(timer->flags & TIMER_IRQSAFE)); + /* + * Must be able to sleep on PREEMPT_RT because of the slowpath in + * del_timer_wait_running(). + */ + if (IS_ENABLED(CONFIG_PREEMPT_RT) && !(timer->flags & TIMER_IRQSAFE)) + lockdep_assert_preemption_enabled(); + do { ret = try_to_del_timer_sync(timer); -- 2.29.1
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