linux-kernel - Re: [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86

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Message-ID: <20180927085728.GF5254@hirez.programming.kicks-ass.net>
Date:   Thu, 27 Sep 2018 10:57:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     will.deacon@....com, mingo@...nel.org,
        linux-kernel@...r.kernel.org, longman@...hat.com,
        tglx@...utronix.de
Subject: Re: [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86

On Thu, Sep 27, 2018 at 10:13:15AM +0200, Andrea Parri wrote:
> On Thu, Sep 27, 2018 at 09:59:35AM +0200, Peter Zijlstra wrote:
> > On Thu, Sep 27, 2018 at 09:47:48AM +0200, Andrea Parri wrote:
> > > > LKMM in particular does _NOT_ deal with mixed sized atomics _at_all_.
> > > 
> > > True, but it is nothing conceptually new to deal with: there're Cat
> > > models that handle mixed-size accesses, just give it time.
> > 
> > Sure, but until that time I must not rely on (and thus not use) LKMM for
> > qspinlock things.
> 
> This is way too generic to be agreed ;D

Only if you know that thing well enough to know why it gives a certain
answer, and thus don't already need it.

If you need it to help you with something; it can't because it doesn't
do the mixed size thing.

> > So while your argument about coherence might be true -- I'll have to
> > think about it; litmus tests are out the window.
> 
> You trimmed the litmus test I gave you.

Because of not wanting to reverse engineer the argument from the litmus
test. But yes, I think I see your point, the earlier trylock will load
the new value and our later load cannot be earlier than that.