From: "tip-bot2 for Thomas Gleixner" <tip-bot2@linutronix.de>
To: linux-tip-commits@vger.kernel.org
Cc: syzbot+aa7c2385d46c5eba0b89@syzkaller.appspotmail.com,
syzbot+abea4558531bae1ba9fe@syzkaller.appspotmail.com,
Thomas Gleixner <tglx@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
stable@vger.kernel.org, x86@kernel.org,
linux-kernel@vger.kernel.org
Subject: [tip: timers/urgent] timers: Move clearing of base::timer_running under base:: Lock
Date: Tue, 27 Jul 2021 19:00:29 -0000 [thread overview]
Message-ID: <162741242945.395.1178547166318427399.tip-bot2@tip-bot2> (raw)
In-Reply-To: <87lfea7gw8.fsf@nanos.tec.linutronix.de>
The following commit has been merged into the timers/urgent branch of tip:
Commit-ID: bb7262b295472eb6858b5c49893954794027cd84
Gitweb: https://git.kernel.org/tip/bb7262b295472eb6858b5c49893954794027cd84
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Sun, 06 Dec 2020 22:40:07 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Tue, 27 Jul 2021 20:57:44 +02:00
timers: Move clearing of base::timer_running under base:: Lock
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
Frederic identified an issue which is:
int data = 0;
void timer_func(struct timer_list *t)
{
data = 1;
}
CPU 0 CPU 1
------------------------------ --------------------------
base = lock_timer_base(timer, &flags); raw_spin_unlock(&base->lock);
if (base->running_timer != timer) call_timer_fn(timer, fn, baseclk);
ret = detach_if_pending(timer, base, true); base->running_timer = NULL;
raw_spin_unlock_irqrestore(&base->lock, flags); raw_spin_lock(&base->lock);
x = data;
If the timer has previously executed on CPU 1 and then CPU 0 can observe
base->running_timer == NULL and returns, assuming the timer has completed,
but it's not guaranteed on all architectures. The comment for
del_timer_sync() makes that guarantee. Moving the assignment under
base->lock prevents this.
For non-RT kernel it's performance wise 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.
Reported-by: syzbot+aa7c2385d46c5eba0b89@syzkaller.appspotmail.com
Reported-by: syzbot+abea4558531bae1ba9fe@syzkaller.appspotmail.com
Fixes: 030dcdd197d7 ("timers: Prepare support for PREEMPT_RT")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/87lfea7gw8.fsf@nanos.tec.linutronix.de
Cc: stable@vger.kernel.org
---
kernel/time/timer.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 9eb11c2..e3d2c23 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1265,8 +1265,10 @@ static inline void timer_base_unlock_expiry(struct timer_base *base)
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);
}
}
@@ -1457,14 +1459,14 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
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);
}
}
}
prev parent reply other threads:[~2021-07-27 19:00 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-06 21:40 timers: Move clearing of base::timer_running under base::lock Thomas Gleixner
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-bot2 for Thomas Gleixner [this message]
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