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From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Marco Elver <elver@google.com>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Anna-Maria Behnsen <anna-maria@linutronix.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: timers: Move clearing of base::timer_running under base::lock
Date: Sun, 06 Dec 2020 22:40:07 +0100	[thread overview]
Message-ID: <87lfea7gw8.fsf@nanos.tec.linutronix.de> (raw)

syzbot reported KCSAN data races vs. timer_base::timer_running being set to
NULL without holding base::lock in expire_timers().

This looks innocent and most reads are clearly not problematic but for a
non-RT kernel it's completely irrelevant whether the store happens before
or after taking the lock. For an RT kernel moving the store under the lock
requires an extra unlock/lock pair in the case that there is a waiter for
the timer. But that's not the end of the world and definitely not worth the
trouble of adding boatloads of comments and annotations to the code. Famous
last words...

Reported-by: syzbot+aa7c2385d46c5eba0b89@syzkaller.appspotmail.com
Reported-by: syzbot+abea4558531bae1ba9fe@syzkaller.appspotmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/time/timer.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1263,8 +1263,10 @@ static inline void timer_base_unlock_exp
 static void timer_sync_wait_running(struct timer_base *base)
 {
 	if (atomic_read(&base->timer_waiters)) {
+		raw_spin_unlock_irq(&base->lock);
 		spin_unlock(&base->expiry_lock);
 		spin_lock(&base->expiry_lock);
+		raw_spin_lock_irq(&base->lock);
 	}
 }
 
@@ -1448,14 +1450,14 @@ static void expire_timers(struct timer_b
 		if (timer->flags & TIMER_IRQSAFE) {
 			raw_spin_unlock(&base->lock);
 			call_timer_fn(timer, fn, baseclk);
-			base->running_timer = NULL;
 			raw_spin_lock(&base->lock);
+			base->running_timer = NULL;
 		} else {
 			raw_spin_unlock_irq(&base->lock);
 			call_timer_fn(timer, fn, baseclk);
+			raw_spin_lock_irq(&base->lock);
 			base->running_timer = NULL;
 			timer_sync_wait_running(base);
-			raw_spin_lock_irq(&base->lock);
 		}
 	}
 }

             reply	other threads:[~2020-12-06 21:41 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-06 21:40 Thomas Gleixner [this message]
2020-12-07  1:10 ` Frederic Weisbecker
2020-12-07 12:25   ` Peter Zijlstra
2020-12-07 12:49     ` Frederic Weisbecker
2020-12-07 13:07 ` Sebastian Andrzej Siewior
2020-12-07 14:29   ` Thomas Gleixner
2020-12-07 15:25     ` Sebastian Andrzej Siewior
2020-12-07 16:06       ` Paul E. McKenney
2020-12-08  8:50         ` Sebastian Andrzej Siewior
2020-12-08 15:04           ` Paul E. McKenney
2020-12-11 14:36           ` Thomas Gleixner
2020-12-11 15:04             ` Sebastian Andrzej Siewior
2021-07-27 19:00 ` [tip: timers/urgent] timers: Move clearing of base::timer_running under base:: Lock tip-bot2 for Thomas Gleixner

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