Re: timers: Move clearing of base::timer_running under base::lock

[Frederic Weisbecker]

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.