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

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Message-ID: <20160831072041.GA10138@twins.programming.kicks-ass.net>
Date:   Wed, 31 Aug 2016 09:20:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        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 Wed, Aug 31, 2016 at 07:25:01AM +1000, Benjamin Herrenschmidt wrote:
> 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),

spin-unlock is _not_ a write barrier, its a RELEASE barrier, and is not
sufficient for this.

> 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.

There is enough, you just need to pair the RELEASE with an ACQUIRE to
get a full load-store barrier.