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

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Message-ID: <YFH9Pw3kwCZC1UTB@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Mar 2021 13:59:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-tip-commits@...r.kernel.org,
        Waiman Long <longman@...hat.com>,
        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 12:38:22PM -0000, tip-bot2 for Waiman Long wrote:
> The following commit has been merged into the locking/urgent branch of tip:
> 
> Commit-ID:     5de2055d31ea88fd9ae9709ac95c372a505a60fa
> Gitweb:        https://git.kernel.org/tip/5de2055d31ea88fd9ae9709ac95c372a505a60fa
> Author:        Waiman Long <longman@...hat.com>
> AuthorDate:    Tue, 16 Mar 2021 11:31:16 -04:00
> Committer:     Ingo Molnar <mingo@...nel.org>
> CommitterDate: Wed, 17 Mar 2021 09:56:44 +01:00
> 
> locking/ww_mutex: Simplify use_ww_ctx & ww_ctx handling
> 
> The use_ww_ctx flag is passed to mutex_optimistic_spin(), but the
> function doesn't use it. The frequent use of the (use_ww_ctx && ww_ctx)
> combination is repetitive.
> 
> In fact, ww_ctx should not be used at all if !use_ww_ctx.  Simplify
> ww_mutex code by dropping use_ww_ctx from mutex_optimistic_spin() an
> clear ww_ctx if !use_ww_ctx. In this way, we can replace (use_ww_ctx &&
> ww_ctx) by just (ww_ctx).

The reason this code was like this is because GCC could constant
propagage use_ww_ctx but could not do the same for ww_ctx (since that's
external).

Please double check generated code to make sure you've not introduced a
bunch of extra branches.