From: Peter Zijlstra <peterz@infradead.org>
To: Uros Bizjak <ubizjak@gmail.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
Will Deacon <will@kernel.org>, Waiman Long <longman@redhat.com>,
Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH] locking/osq_lock: Use atomic_try_cmpxchg_release() in osq_unlock()
Date: Fri, 25 Oct 2024 10:02:00 +0200 [thread overview]
Message-ID: <20241025080200.GD14555@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20241001114606.820277-1-ubizjak@gmail.com>
On Tue, Oct 01, 2024 at 01:45:57PM +0200, Uros Bizjak wrote:
> Replace this pattern in osq_unlock():
>
> atomic_cmpxchg(*ptr, old, new) == old
>
> ... with the simpler and faster:
>
> atomic_try_cmpxchg(*ptr, &old, new)
>
> The x86 CMPXCHG instruction returns success in the ZF flag,
> so this change saves a compare after the CMPXCHG. The code
> in the fast path of osq_unlock() improves from:
>
> 11b: 31 c9 xor %ecx,%ecx
> 11d: 8d 50 01 lea 0x1(%rax),%edx
> 120: 89 d0 mov %edx,%eax
> 122: f0 0f b1 0f lock cmpxchg %ecx,(%rdi)
> 126: 39 c2 cmp %eax,%edx
> 128: 75 05 jne 12f <...>
>
> to:
>
> 12b: 31 d2 xor %edx,%edx
> 12d: 83 c0 01 add $0x1,%eax
> 130: f0 0f b1 17 lock cmpxchg %edx,(%rdi)
> 134: 75 05 jne 13b <...>
>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Thanks!
next prev parent reply other threads:[~2024-10-25 8:02 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-10-01 11:45 [PATCH] locking/osq_lock: Use atomic_try_cmpxchg_release() in osq_unlock() Uros Bizjak
2024-10-02 15:47 ` Waiman Long
2024-10-25 8:02 ` Peter Zijlstra [this message]
2024-10-26 7:35 ` [tip: locking/core] " tip-bot2 for Uros Bizjak
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