linux-kernel - [PATCH RT 5/6] timers: Dont block on ->expiry_lock for TIMER

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Date:   Fri, 06 Nov 2020 21:06:41 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Tom Zanussi <zanussi@...nel.org>,
        "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>,
        Mike Galbraith <efault@....de>, stable-rt@...r.kernel.org
Subject: [PATCH RT 5/6] timers: Dont block on ->expiry_lock for TIMER_IRQSAFE

5.4.74-rt42-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

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@....de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: stable-rt@...r.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 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 89078fd848b9..3e9d7f227a5c 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1289,7 +1289,7 @@ 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);

 		/*
@@ -1373,6 +1373,13 @@ 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))
+		might_sleep();
+
 	do {
 		ret = try_to_del_timer_sync(timer);

-- 
2.28.0