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From:   John Ogness <john.ogness@linutronix.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Petr Mladek <pmladek@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        kexec@lists.infradead.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH v2 2/3] printk: add lockless buffer
References: <20200501094010.17694-1-john.ogness@linutronix.de>
        <20200501094010.17694-3-john.ogness@linutronix.de>
        <87v9ktcs3q.fsf@vostro.fn.ogness.net>
        <CAHk-=whSLRiNxehLiuX+ZfHsu8Kpj7R1Sdr3zA7==SMW0zh3ug@mail.gmail.com>
Date:   Tue, 19 May 2020 22:34:19 +0200
In-Reply-To: <CAHk-=whSLRiNxehLiuX+ZfHsu8Kpj7R1Sdr3zA7==SMW0zh3ug@mail.gmail.com>
        (Linus Torvalds's message of "Mon, 18 May 2020 10:22:30 -0700")
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On 2020-05-18, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>>                 smp_mb(); /* LMM(data_push_tail:C) */
>>
>> +               if (atomic_long_try_cmpxchg_relaxed(&data_ring->tail_lpos,
>> +                               &tail_lpos,
>> +                               next_lpos)) { /* LMM(data_push_tail:D) */
>> +                       break;
>> +               }
>
> Doing an "smp_mb()" followed by a "cmpxchg_relaxed" seems all kinds of
> odd and pointless, and is very much non-optimal on x86 for example.,
>
> Just remove the smp_mb(), and use the non-relaxed form of cmpxchg.
> It's defined to be fully ordered if it succeeds (and if the cmpxchg
> doesn't succeed, it's a no-op and the memory barrier shouldn't make
> any difference).
>
> Otherwise you'll do two memory ordering operations on x86 (and
> probably some other architectures), since the cmpxchg is always
> ordered on x86 and there exists no "relaxed" form of it.

ACK.

All three smp_mb()'s and both smp_wmb()'s sit directly next to
cmpxchg_relaxed() calls. Having explicit memory barriers was helpful for
identifying, proving, and testing a minimal set of pairs (on arm64), but
all will be folded into full cmpxchg()'s for the next version.

John Ogness