linux-kernel - Re: [RFC][PATCH 1/4] sched: Fix a race between __kthread_bind() and sched

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Message-ID: <20150807153828.GE14626@mtj.duckdns.org>
Date:	Fri, 7 Aug 2015 11:38:28 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, riel@...hat.com, dedekind1@...il.com,
	linux-kernel@...r.kernel.org, mgorman@...e.de, rostedt@...dmis.org,
	juri.lelli@....com, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC][PATCH 1/4] sched: Fix a race between __kthread_bind() and
 sched_setaffinity()

Hello,

On Fri, Aug 07, 2015 at 05:29:56PM +0200, Peter Zijlstra wrote:
> Even if we were to strictly order those stores you could have (note
> there is no matching barrier, as there is only the one load, so ordering
> cannot help):
> 
> 	__kthread_bind()
> 						<SYSCALL>
> 						  sched_setaffinity()
> 						    if (p->flags & PF_NO_SETAFFINITY) /* false-not-taken */
> 	  p->flags |= PF_NO_SETAFFINITY;
> 	  smp_wmb();
> 	  do_set_cpus_allowed();
> 						    set_cpus_allowed_ptr()
>
> > I think the code was better before.  Can't we just revert workqueue.c
> > part?
> 
> I agree that the new argument isn't pretty, but I cannot see how
> workqueues would not be affected by this.

So, the problem there is that __kthread_bind() doesn't grab the same
lock that the syscall side grabs but workqueue used
set_cpus_allowed_ptr() which goes through the rq locking, so as long
as the check on syscall side is movied inside rq lock, it should be
fine.

Thanks.

-- 
tejun
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