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>

Author: ubizjak

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);
 	}