linux-kernel - Re: [PATCH] fs/btrfs: Use atomic_try_cmpxchg in free_extent

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Message-ID: <20220818171414.GM13489@twin.jikos.cz>
Date:   Thu, 18 Aug 2022 19:14:14 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>
Subject: Re: [PATCH] fs/btrfs: Use atomic_try_cmpxchg in free_extent_buffer

On Tue, Aug 09, 2022 at 06:36:33PM +0200, Uros Bizjak wrote:
> Use `atomic_try_cmpxchg(ptr, &old, new)` instead of
> `atomic_cmpxchg(ptr, old, new) == old` in free_extent_buffer. This
> has two benefits:
> 
> - The x86 cmpxchg instruction returns success in the ZF flag, so this
>   change saves a compare after cmpxchg, as well as a related move
>   instruction in the front of cmpxchg.
> 
> - atomic_try_cmpxchg implicitly assigns the *ptr value to &old when
>   cmpxchg fails, enabling further code simplifications.
> 
> This patch has no functional change.
> 
> Cc: Chris Mason <clm@...com>
> Cc: Josef Bacik <josef@...icpanda.com>
> Cc: David Sterba <dsterba@...e.com>
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>

It's not in a performance critical code but resulting code is slightly
more efficient. Added to misc-next, thanks.