linux-kernel - Re: [RFC][PATCH] Fix a race between rwsem and the scheduler

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Message-ID: <1472592301.2388.37.camel@kernel.crashing.org>
Date:   Wed, 31 Aug 2016 07:25:01 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Balbir Singh <bsingharora@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <nicholas.piggin@...il.com>
Subject: Re: [RFC][PATCH] Fix a race between rwsem and the scheduler

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.