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From: Frederic Weisbecker <frederic@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Marco Elver <elver@google.com>,
	kasan-dev <kasan-dev@googlegroups.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Anna-Maria Behnsen <anna-maria@linutronix.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock
Date: Mon, 7 Dec 2020 13:49:15 +0100	[thread overview]
Message-ID: <20201207124915.GA122233@lothringen> (raw)
In-Reply-To: <20201207122513.GT3021@hirez.programming.kicks-ass.net>

On Mon, Dec 07, 2020 at 01:25:13PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 07, 2020 at 02:10:13AM +0100, Frederic Weisbecker wrote:
> > On Sun, Dec 06, 2020 at 10:40:07PM +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...
> > 
> > There is another thing I noticed lately wrt. del_timer_sync() VS timer execution:
> 
> > Here if the timer has previously executed on CPU 1 and then CPU 0 sees base->running_timer == NULL,
> > it will return, assuming the timer has completed. But there is nothing to enforce the fact that x
> > will be equal to 1. Enforcing that is a behaviour I would expect in this case since this is a kind
> > of "wait for completion" function. But perhaps it doesn't apply here, in fact I have no idea...
> > 
> > But if we recognize that as an issue, we would need a mirroring load_acquire()/store_release() on
> > base->running_timer.
> 
> Yeah, I think you're right. del_timer_sync() explicitly states it waits
> for completion of the handler, so it isn't weird to then also expect to
> be able to observe the results of the handler.
> 
> Thomas' patch fixes this.

Indeed!

Thanks.

  reply	other threads:[~2020-12-07 12:50 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-06 21:40 Thomas Gleixner
2020-12-07  1:10 ` Frederic Weisbecker
2020-12-07 12:25   ` Peter Zijlstra
2020-12-07 12:49     ` Frederic Weisbecker [this message]
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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