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 <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Stefano Stabellini <[email protected]>
Cc: Oleksandr Tyshchenko <[email protected]>
---
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);
}
--
2.52.0
