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

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Message-ID: <20160831071810.GZ10138@twins.programming.kicks-ass.net>
Date:   Wed, 31 Aug 2016 09:18:10 +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 07:28:18AM +1000, Benjamin Herrenschmidt wrote:
> It's always been a requirement that if you actually context switch a
> full mb() is implied ...

> On powerpc we have a sync deep in _switch to achieve that.

OK, fair enough. I must've missed it in the x86 switch_to, must be one
of those implied serializing instructions I'm not too familiar with.

> (though that isn't the case if you don't actually
> switch, ie, you are back to RUNNING before you even hit schedule).

Right, which invalidates the claim that schedule() implies a full mb,

> This is necessary so that a process who wakes up on a different CPU sees
> all of its own load/stores.

Don't actually think its needed for that, see the comment from
8643cda549ca4, the scheduler has enough barriers to guarantee
Program-Order for tasks without that.