Re: [PATCH] xen/mcelog: simplify MCE_GETCLEAR_FLAGS using xchg()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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@xxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> ---
>  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@xxxxxxxxxx>