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Subject: Re: [RFC][PATCH] Fix a race between rwsem and the scheduler
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Oleg Nesterov <oleg@redhat.com>, Peter Zijlstra <peterz@infradead.org>
Cc: Balbir Singh <bsingharora@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Nicholas Piggin <nicholas.piggin@gmail.com>
Date: Wed, 31 Aug 2016 07:25:01 +1000
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On Tue, 2016-08-30 at 15:04 +0200, Oleg Nesterov wrote:
> 
> Confused... how this connects to UNLOCK+LOCK on rq->lock? A LOAD can
> leak into the critical section.
> 
> But context switch should imply mb() we can rely on?

Between setting of ->on_rq and returning to the task so it can
change its state back to [UN]INTERRUPTIBLE, there will be at least one
write barrier (spin unlock of the rq), possibly even a full barrier
(context switch). The write barrier is enough so I didn't dig to make
sure we always context switch in the scenario we're looking at but I
think we do.

Cheers,
Ben.