linux-kernel - Re: timers: Move clearing of base::timer

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Message-ID: <20201207130753.kpxf2ydroccjzrge@linutronix.de>
Date:   Mon, 7 Dec 2020 14:07:53 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Marco Elver <elver@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock

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@...kaller.appspotmail.com
> Reported-by: syzbot+abea4558531bae1ba9fe@...kaller.appspotmail.com
> Signed-off-by: Thomas Gleixner <tglx@...utronix.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.

Sebastian