linux-kernel - Re: [tip: locking/urgent] locking/ww_mutex: Simplify use_ww_ctx & ww

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Message-ID: <YFIPYmFE7ChGrpf2@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Mar 2021 15:17:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        Davidlohr Bueso <dbueso@...e.de>, x86@...nel.org
Subject: Re: [tip: locking/urgent] locking/ww_mutex: Simplify use_ww_ctx &
 ww_ctx handling

On Wed, Mar 17, 2021 at 10:10:16AM -0400, Waiman Long wrote:
> On 3/17/21 9:55 AM, Peter Zijlstra wrote:
> > On Wed, Mar 17, 2021 at 09:43:20AM -0400, Waiman Long wrote:
> > 
> > > Using gcc 8.4.1, the generated __mutex_lock function has the same size (with
> > > last instruction at offset +5179) with or without this patch. Well, you can
> > > say that this patch is an no-op wrt generated code.
> > OK, then GCC has gotten better. Because back then I tried really hard
> > but it wouldn't remove the if (ww_ctx) branches unless I had that extra
> > const bool argument.
> > 
> I think ww_mutex was merged in 2013. That is almost 8 years ago. It could
> still be the case that older gcc compilers may not generate the right code.
> I will try the RHEL7 gcc compiler (4.8.5) to see how it fares.

I really don't care about code generation qualitee of anything before
8-ish at this point. That's already an old compiler.

If you run on ancient compilers, you simply don't care about code
quality.