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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.
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