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Date: Tue, 30 Aug 2016 16:13:21 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Balbir Singh <bsingharora@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Nicholas Piggin <nicholas.piggin@gmail.com>
Subject: Re: [RFC][PATCH] Fix a race between rwsem and the scheduler
Message-ID: <20160830141321.GB2794@worktop>
References: <4050f2ce-1aee-d2aa-39e3-36e995b56252@gmail.com>
 <20160830121937.GQ10138@twins.programming.kicks-ass.net>
 <20160830130426.GA17795@redhat.com>
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On Tue, Aug 30, 2016 at 03:04:27PM +0200, Oleg Nesterov wrote:
> On 08/30, Peter Zijlstra wrote:
> >
> > 	/*
> > 	 * Ensure we load p->on_rq _after_ p->state, otherwise it would
> > 	 * be possible to, falsely, observe p->on_rq == 0 and get stuck
> > 	 * in smp_cond_load_acquire() below.
> > 	 *
> > 	 * sched_ttwu_pending()			try_to_wake_up()
> > 	 *   [S] p->on_rq = 1;			[L] P->state
> > 	 *       UNLOCK rq->lock
> > 	 *
> > 	 * schedule()				RMB
> > 	 *       LOCK rq->lock
> > 	 *       UNLOCK rq->lock
> > 	 *
> > 	 * [task p]
> > 	 *   [S] p->state = UNINTERRUPTIBLE	[L] p->on_rq
> > 	 *
> > 	 * Pairs with the UNLOCK+LOCK on rq->lock from the
> > 	 * last wakeup of our task and the schedule that got our task
> > 	 * current.
> > 	 */
> 
> Confused... how this connects to UNLOCK+LOCK on rq->lock? A LOAD can
> leak into the critical section.

How so? That LOCK+UNLOCK which is leaky, UNLOCK+LOCK is a read/write
barrier (just not an MB because it lacks full transitivity).

> But context switch should imply mb() we can rely on?

Not sure it should, on x86 switch_mm does a CR3 write and that is
serializing, but switch_to() doesn't need to do anything iirc.