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

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Message-ID: <bbfca577-b680-4c73-3f35-22179bd1a498@redhat.com>
Date:   Wed, 17 Mar 2021 10:10:16 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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.

Cheers,
Longman