linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Fri, 12 Dec 2014 12:34:18 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Lang <david@...g.hm>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Fri, Dec 12, 2014 at 11:58:50AM -0800, David Lang wrote:
> On Fri, 12 Dec 2014, Linus Torvalds wrote:
> 
> >I'm also not sure if the bug ever happens with preemption disabled.
> >Sasha, was that you who reported that you cannot reproduce it without
> >preemption? It strikes me that there's a race condition in
> >__cond_resched() wrt preemption, for example: we do
> >
> >       __preempt_count_add(PREEMPT_ACTIVE);
> >       __schedule();
> >       __preempt_count_sub(PREEMPT_ACTIVE);
> >
> >and in between the __schedule() and __preempt_count_sub(), if an
> >interrupt comes in and wakes up some important process, it won't
> >reschedule (because preemption is active), but then we enable
> >preemption again and don't check whether we should reschedule (again),
> >and we just go on our merry ways.
> >
> >Now, I don't see how that could really matter for a long time -
> >returning to user space will check need_resched, and sleeping will
> >obviously force a reschedule anyway, so these kinds of races should at
> >most delay things by just a tiny amount,
> 
> If the machine has NOHZ and has a cpu bound userspace task, it could
> take quite a while before userspace would trigger a reschedule (at
> least if I've understood the comments on this thread properly)

Dave, Sasha, if you guys are running CONFIG_NO_HZ_FULL=y and
CONFIG_NO_HZ_FULL_ALL=y, please let me know.  I am currently assuming
that none of your CPUs are in NO_HZ_FULL mode.  If this assumption is
incorrect, there are some other pieces of RCU that I should be taking
a hard look at.

							Thanx, Paul

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