Re: Crashes with 874bbfe600a6 in 3.18.25

[Mike Galbraith]

On Thu, 2016-02-04 at 11:46 +0100, Thomas Gleixner wrote:
> On Thu, 4 Feb 2016, Mike Galbraith wrote:

> > I'm also wondering why 22b886dd only applies to kernels >= 4.2.
> > 
> > 
> > Regardless of the previous CPU a timer was on, add_timer_on()
> > currently simply sets timer->flags to the new CPU.  As the caller must
> > be seeing the timer as idle, this is locally fine, but the timer
> > leaving the old base while unlocked can lead to race conditions as
> > follows.
> > 
> > Let's say timer was on cpu 0.
> > 
> >   cpu 0                                 cpu 1
> >   -----------------------------------------------------------------------------
> >   del_timer(timer) succeeds
> >                                         del_timer(timer)
> >                                           lock_timer_base(timer) locks cpu_0_base
> >   add_timer_on(timer, 1)
> >     spin_lock(&cpu_1_base->lock)
> >     timer->flags set to cpu_1_base
> >     operates on @timer                    operates on @timer
> > 
> > 
> > What's the difference between...
> >      timer->flags = (timer->flags & ~TIMER_BASEMASK) | cpu;
> > and...
> >      timer_set_base(timer, base);
> > 
> > ...that makes that fix unneeded prior to 4.2?  We take the same locks
> > in < 4.2 kernels, so seemingly both will diddle concurrently above.
> 
> Indeed, you are right.

Whew, thanks for confirming, looking for what the hell I was missing
wasn't going well at all, ate most of my day.

> The same can happen on pre 4.2, just the fix does not apply as we changed the
> internals how the base is managed in the timer itself. Backport below.

Exactly what I did locally.

	-Mike