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Date: Fri, 7 Aug 2015 11:38:28 -0400
From: Tejun Heo <tj@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, riel@redhat.com, dedekind1@gmail.com,
        linux-kernel@vger.kernel.org, mgorman@suse.de, rostedt@goodmis.org,
        juri.lelli@arm.com, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [RFC][PATCH 1/4] sched: Fix a race between __kthread_bind() and
 sched_setaffinity()
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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