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From: Thomas Gleixner <tglx@linutronix.de>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	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>
Subject: Re: timers: Move clearing of base::timer_running under base::lock
Date: Mon, 07 Dec 2020 15:29:50 +0100	[thread overview]
Message-ID: <87a6up7kpt.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20201207130753.kpxf2ydroccjzrge@linutronix.de>

On Mon, Dec 07 2020 at 14:07, Sebastian Andrzej Siewior wrote:
> On 2020-12-06 22:40:07 [+0100], Thomas Gleixner wrote:
>> 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>
>
> One thing I noticed while testing it is that the "corner" case in
> timer_sync_wait_running() is quite reliably hit by rcu_preempt
> rcu_gp_fqs_loop() -> swait_event_idle_timeout_exclusive() invocation.

I assume it's something like this:

     timeout -> wakeup

->preemption
        del_timer_sync()
                .....

Thanks,

        tglx

  reply	other threads:[~2020-12-07 14:30 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 [this message]
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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