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

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Message-ID: <b8262c0d-5602-436f-9137-46621e39fd03@citrix.com>
Date: Thu, 22 Jan 2026 14:22:01 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Uros Bizjak <ubizjak@...il.com>, xen-devel@...ts.xenproject.org,
 linux-kernel@...r.kernel.org
Cc: Andrew Cooper <andrew.cooper3@...rix.com>, Juergen Gross
 <jgross@...e.com>, Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH] xen/mcelog: simplify MCE_GETCLEAR_FLAGS using xchg()

On 22/01/2026 2:17 pm, 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>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Stefano Stabellini <sstabellini@...nel.org>
> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
> ---
>  drivers/xen/mcelog.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/drivers/xen/mcelog.c b/drivers/xen/mcelog.c
> index 4f65b641c054..abe658c73b7b 100644
> --- a/drivers/xen/mcelog.c
> +++ b/drivers/xen/mcelog.c
> @@ -165,9 +165,7 @@ static long xen_mce_chrdev_ioctl(struct file *f, unsigned int cmd,
>  	case MCE_GETCLEAR_FLAGS: {
>  		unsigned flags;
>  
> -		do {
> -			flags = xen_mcelog.flags;
> -		} while (cmpxchg(&xen_mcelog.flags, flags, 0) != flags);
> +		flags = xchg(&xen_mcelog.flags, 0);
>  
>  		return put_user(flags, p);
>  	}

Oh yeah, that was very daft beforehand.

Reviewed-by: Andrew Cooper <andrew.cooper3@...rix.com>