From: Andrew Morton <akpm@linux-foundation.org>
To: Uros Bizjak <ubizjak@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] lib/genalloc: use try_cmpxchg in {set,clear}_bits_ll
Date: Wed, 18 Jan 2023 13:18:25 -0800 [thread overview]
Message-ID: <20230118131825.c6daea81ea1e2dc6aa014f38@linux-foundation.org> (raw)
In-Reply-To: <20230118150703.4024-1-ubizjak@gmail.com>
On Wed, 18 Jan 2023 16:07:03 +0100 Uros Bizjak <ubizjak@gmail.com> wrote:
> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
> {set,clear}_bits_ll. x86 CMPXCHG instruction returns success in ZF
> flag, so this change saves a compare after cmpxchg (and related move
> instruction in front of cmpxchg).
>
> Also, try_cmpxchg implicitly assigns old *ptr value to "old"
> when cmpxchg fails.
>
> Note that the value from *ptr should be read using READ_ONCE to prevent
> the compiler from merging, refetching or reordering the read.
>
> The patch also declares these two functions inline, to ensure inlining.
But why is that better? This adds a few hundred bytes more text, which
has a cost.
next prev parent reply other threads:[~2023-01-18 21:18 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-18 15:07 [PATCH] lib/genalloc: use try_cmpxchg in {set,clear}_bits_ll Uros Bizjak
2023-01-18 21:18 ` Andrew Morton [this message]
2023-01-18 21:47 ` Uros Bizjak
2023-01-18 21:55 ` Uros Bizjak
2023-01-18 22:01 ` Uros Bizjak
2023-01-19 12:47 ` David Laight
2023-01-23 15:04 ` Uros Bizjak
2023-01-23 15:42 ` David Laight
2023-01-23 15:59 ` Mateusz Guzik
2023-01-23 19:36 ` Linus Torvalds
2023-01-24 0:11 ` Mateusz Guzik
2023-01-24 1:51 ` Linus Torvalds
2023-01-24 6:48 ` Mateusz Guzik
2023-01-24 8:54 ` David Laight
2023-01-24 16:58 ` Linus Torvalds
2023-01-24 17:32 ` David Laight
2023-01-27 3:54 ` Al Viro
2023-01-27 19:25 ` Linus Torvalds
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