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

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Date:   Tue, 30 Aug 2016 16:13:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Balbir Singh <bsingharora@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Nicholas Piggin <nicholas.piggin@...il.com>
Subject: Re: [RFC][PATCH] Fix a race between rwsem and the scheduler

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.