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

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Date:   Wed, 31 Aug 2016 20:56:17 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.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, 2016-08-31 at 09:18 +0200, Peter Zijlstra wrote:
> 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,

Right, it's only full mb if you actually schedule to another process :-
)

> > 
> > 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.
>