linux-kernel - Re: [PATCH v2 RESEND] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86

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Date:   Sun, 3 Jul 2022 14:01:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 RESEND] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF
 for X86_CMPXCHG64

On Sun, Jun 26, 2022 at 1:18 PM Uros Bizjak <ubizjak@...il.com> wrote:
>
>    Also, by using try_cmpxchg64() instead of cmpxchg64()
> in CMPXCHG_LOOP macro, the compiler actually produces sane code,
> improving lockref_get_or_lock main loop from:

Heh. I'm actually looking at that function because I committed my "add
sparse annotation for conditional locking" patch, and
lockref_get_or_lock() has the wrong "polarity" for conditional locking
(it returns false when it takes the lock).

But then I started looking closer, and that function has no users any
more. In fact, it hasn't had users since back in 2013.

So while I still think ARCH_USE_CMPXCHG_LOCKREF is fine for 32-bit
x86, the part about improving lockref_get_or_lock() code generation is
kind of pointless. I'm going to remove that function as "unused, and
with the wrong return value".

              Linus