xen/mcelog: simplify MCE_GETCLEAR_FLAGS using xchg()

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@gmail.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20260122141754.116129-1-ubizjak@gmail.com>

diff --git a/drivers/xen/mcelog.c b/drivers/xen/mcelog.c
index 4f65b641c054551ffb2498e90ba6385672af2d34..abe658c73b7b026503f3d973988efc81aae7de94 100644 (file)
--- 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);
        }