linux-kernel - Re: [RFC][PATCH 5/5] powerpc: Remove SYNC from

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Message-ID: <20170608065400.zhfao5lba6i3s7j6@hirez.programming.kicks-ass.net>
Date:   Thu, 8 Jun 2017 08:54:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     torvalds@...ux-foundation.org, will.deacon@....com,
        oleg@...hat.com, paulmck@...ux.vnet.ibm.com,
        benh@...nel.crashing.org, mpe@...erman.id.au,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        stern@...land.harvard.edu
Subject: Re: [RFC][PATCH 5/5] powerpc: Remove SYNC from _switch

On Thu, Jun 08, 2017 at 10:32:44AM +1000, Nicholas Piggin wrote:
> On Wed, 07 Jun 2017 18:15:06 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Now that the scheduler's rq->lock is RCsc and thus provides full
> > transitivity between scheduling actions. And since we cannot migrate
> > current, a task needs a switch-out and a switch-in in order to
> > migrate, in which case the RCsc provides all the ordering we need.
> 
> Hi Peter,
> 
> I'm actually just working on removing this right now too, so
> good timing.
> 
> I think we can't "just" remove it, because it is required to order
> MMIO on powerpc as well.

How is MMIO special? That is, there is only MMIO before we call into
schedule() right? So the rq->lock should be sufficient to order that
too.

> 
> But what I have done is to comment that some other primitives are
> already providing the hwsync for other, so we don't have to add
> another one in _switch.

Right, so this patch relies on the smp_mb__before_spinlock ->
smp_mb__after_spinlock conversion that makes the rq->lock RCsc and
should thus provide the required SYNC for migrations.

That said, I think you can already use the smp_mb__before_spinlock() as
that is done with IRQs disabled, but its a more difficult argument. The
rq->lock RCsc property should be more obvious.