linux-kernel - Re: [PATCH 3/3] ring_buffer: Use try

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Message-ID: <CAFULd4aQTEmJz6Q9B6vSm-90A41g7T1pVntaR_PUMSoFRQUGZg@mail.gmail.com>
Date:   Wed, 1 Mar 2023 18:57:13 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 3/3] ring_buffer: Use try_cmpxchg instead of cmpxchg

On Wed, Mar 1, 2023 at 5:28 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 1 Mar 2023 11:18:50 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
>
> > But with gcc 12.2.0 I don't really see the benefit. And I'm worried that
> > the side effect of modifying the old variable could cause a bug in the
> > future, if it is used after the try_cmpxchg(). At least for the second case.
>
> Actually, I like Joel's recommendation of adding a cmpxchg_succeeded()
> function, that does the try_cmpxchg() without needing to save the old
> variable. That's my main concern, as it does have that side effect that
> could be missed when updating the code.

The "controversial" part of the patch would then look like the
attached patch. As expected, the compiler again produces expected
code:

     eb8:    48 8b 0e                 mov    (%rsi),%rcx
     ebb:    48 83 e1 fc              and    $0xfffffffffffffffc,%rcx
     ebf:    48 83 c9 01              or     $0x1,%rcx
     ec3:    48 89 c8                 mov    %rcx,%rax
     ec6:    f0 48 0f b1 3e           lock cmpxchg %rdi,(%rsi)
     ecb:    48 39 c1                 cmp    %rax,%rcx
     ece:    74 2d                    je     efd <rb_get_reader_page+0x12d>

to:

     eb8:    48 8b 01                 mov    (%rcx),%rax
     ebb:    48 83 e0 fc              and    $0xfffffffffffffffc,%rax
     ebf:    48 83 c8 01              or     $0x1,%rax
     ec3:    f0 48 0f b1 31           lock cmpxchg %rsi,(%rcx)
     ec8:    74 2d                    je     ef7 <rb_get_reader_page+0x127>

Uros.

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