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

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Date:   Mon, 7 Dec 2020 02:10:13 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
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>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock

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:


    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;
    

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.