linux-kernel - Re: Race condition in HR timers that cause double insertion and hard lockup -- all latest versions

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Date:	Tue, 2 Sep 2014 21:08:31 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Itzcak Pechtalt <itzcak@...shnetworks.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Race condition in HR timers that cause double insertion and hard
 lockup -- all latest versions

On Tue, 2 Sep 2014, Linus Torvalds wrote:

> On Tue, Sep 2, 2014 at 8:45 AM, Itzcak Pechtalt
> <itzcak@...shnetworks.com> wrote:
> >
> > I opened a bug in https://bugzilla.kernel.org/show_bug.cgi?id=83601  for this subject with full description.
> > There is also a short fix patch for kernel/hrtimer.c file.
> > Even if this bug occurs rary, however it resolves system hard lockup option.
> 
> The patch is whitespace-damaged, but with a small oneliner like this
> that doesn't much matter (the timer files moved to kernel/time/ during
> this merge window, so the patch wouldn't apply as-is anyway).
> 
> It needs a sign-off (see Documentation/SubmittingPatches), but even
> more importantly it needs to go to the right people for
> double-checking.
> 
> But the patch is more broken than whitespace and even lack of
> sign-off. It cannot even have compiled. I'm assuming "timer_state" was
> intended to be "timer->state". Also, every caller but one already has
> "HRTIMER_STATE_CALLBACK" set unconditionally or to the old state in
> "newstate", so I suspect if this patch is the real fix (which I'll
> leave for Thomas to comment more on), afaik the actual problem can
> only happen through migrate_hrtimer_list() which uconditionally sets
> the whole state to HRTIMER_STATE_MIGRATE.
> 
> Thomas? Leaving damaged patch quoted below.

Right. It's been fixed long ago and the migrate path cannot suffer
from this problem because at this point a callback running on the dead
cpu would cause the 

    BUG_ON(hrtimer_callback_running(timer));

a few lines above the remove_hrtimer() call to trigger and send the
machine into lala land.

Thanks,

	tglx
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