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From: John Ogness <john.ogness@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrea Parri <parri.andrea@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	kexec@lists.infradead.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2 2/3] printk: add lockless buffer
Date: Tue, 19 May 2020 22:34:19 +0200	[thread overview]
Message-ID: <87tv0bhdf8.fsf@vostro.fn.ogness.net> (raw)
In-Reply-To: <CAHk-=whSLRiNxehLiuX+ZfHsu8Kpj7R1Sdr3zA7==SMW0zh3ug@mail.gmail.com> (Linus Torvalds's message of "Mon, 18 May 2020 10:22:30 -0700")

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

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  reply	other threads:[~2020-05-19 20:34 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-01  9:40 [PATCH v2 0/3] printk: replace ringbuffer John Ogness
2020-05-01  9:40 ` [PATCH v2 1/3] crash: add VMCOREINFO macro for anonymous structs John Ogness
2020-06-03 10:16   ` Petr Mladek
2020-05-01  9:40 ` [PATCH v2 2/3] printk: add lockless buffer John Ogness
2020-05-18 13:03   ` John Ogness
2020-05-18 17:22     ` Linus Torvalds
2020-05-19 20:34       ` John Ogness [this message]
2020-06-09  7:10   ` blk->id read race: was: " Petr Mladek
2020-06-09 14:18     ` John Ogness
2020-06-10  8:42       ` Petr Mladek
2020-06-10 13:55         ` John Ogness
2020-06-09  9:31   ` redundant check in make_data_reusable(): was " Petr Mladek
2020-06-09 14:48     ` John Ogness
2020-06-10  9:38       ` Petr Mladek
2020-06-10 10:24         ` John Ogness
2020-06-10 14:56           ` John Ogness
2020-06-11 19:51             ` John Ogness
2020-06-11 13:55           ` Petr Mladek
2020-06-11 20:25             ` John Ogness
2020-06-09  9:48   ` Full barrier in data_push_tail(): " Petr Mladek
2020-06-09 15:03     ` John Ogness
2020-06-09 11:37   ` Barrier before pushing desc_ring tail: " Petr Mladek
2020-06-09 15:56     ` John Ogness
2020-06-11 12:01       ` Petr Mladek
2020-06-11 23:06         ` John Ogness
2020-06-09 14:38   ` data_ring head_lpos and tail_lpos synchronization: " Petr Mladek
2020-06-10  7:53     ` John Ogness
2020-05-01  9:40 ` [PATCH v2 3/3] printk: use the lockless ringbuffer John Ogness
2020-05-06 14:50   ` John Ogness
2020-05-13 12:05 ` [PATCH v2 0/3] printk: replace ringbuffer Prarit Bhargava
2020-05-15 10:24 ` Sergey Senozhatsky

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