linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Fri, 12 Dec 2014 11:58:50 -0800 (PST)
From:	David Lang <david@...g.hm>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

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)

David Lang
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