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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 > Cc: Juergen Gross > Cc: Stefano Stabellini > Cc: Oleksandr Tyshchenko > --- > 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