linux-kernel - Re: [PATCH] xen/mcelog: simplify MCE_GETCLEAR

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Message-ID: <17b237ae-d3c8-4491-8eb5-6ebb4a25a8fc@suse.com>
Date: Thu, 22 Jan 2026 15:21:53 +0100
From: Jan Beulich <jbeulich@...e.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Juergen Gross <jgross@...e.com>,
 Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xen/mcelog: simplify MCE_GETCLEAR_FLAGS using xchg()

On 22.01.2026 15:17, Uros Bizjak wrote:
> The MCE_GETCLEAR_FLAGS ioctl retrieves xen_mcelog.flags while
> atomically clearing it. This was previously implemented using a
> cmpxchg() loop.
> 
> Replace the cmpxchg() loop with a single xchg(), which provides the
> same atomic get-and-clear semantics, avoids retry spinning under
> contention, and simplifies the code.
> 
> The code on x86_64 improves from:
> 
>     186:	8b 15 00 00 00 00    	mov    0x0(%rip),%edx
>     18c:	89 d0                	mov    %edx,%eax
>     18e:	f0 0f b1 0d 00 00 00 	lock cmpxchg %ecx,0x0(%rip)
>     195:	00
>     196:	39 c2                	cmp    %eax,%edx
>     198:	75 ec                	jne    186 <...>
> 
> to just:
> 
>     186:	87 05 00 00 00 00    	xchg   %eax,0x0(%rip)
> 
> No functional change intended.
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>

Reviewed-by: Jan Beulich <jbeulich@...e.com>