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

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Message-ID: <20201207124915.GA122233@lothringen>
Date:   Mon, 7 Dec 2020 13:49:15 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Marco Elver <elver@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock

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.