linux-kernel - Re: [PATCH] lib/genalloc: use try_cmpxchg in {set,clear}_bits

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Message-Id: <20230118131825.c6daea81ea1e2dc6aa014f38@linux-foundation.org>
Date:   Wed, 18 Jan 2023 13:18:25 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/genalloc: use try_cmpxchg in {set,clear}_bits_ll

On Wed, 18 Jan 2023 16:07:03 +0100 Uros Bizjak <ubizjak@...il.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.