linux-kernel - Re: [PATCH 2/8] powerpc/pseries: use smp_rmb() in H

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Message-Id: <1593685552.uh4kepm08t.astroid@bobo.none>
Date:   Thu, 02 Jul 2020 20:36:27 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Anton Blanchard <anton@...abs.org>,
        Boqun Feng <boqun.feng@...il.com>, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 2/8] powerpc/pseries: use smp_rmb() in H_CONFER spin yield

Excerpts from Peter Zijlstra's message of July 2, 2020 6:28 pm:
> On Thu, Jul 02, 2020 at 05:48:33PM +1000, Nicholas Piggin wrote:
>> There is no need for rmb(), this allows faster lwsync here.
> 
> Since you determined this; I'm thinking you actually understand the
> ordering here. How about recording this understanding in a comment?
> 
> Also, should the lock->slock load not use READ_ONCE() ?

Yeah, good point. Maybe I'll drop it from this series, doesn't really 
belong I just saw the cleanup and didn't want to forget it.

We we just ordering the two loads in this function, and !SMP isn't a 
concern (i.e., no issues of !SMP guest on SMP HV), but yeah fixing
the lack of comment is warranted, thanks.

Thanks,
Nick