2022-12-21 19:45:02

by Uros Bizjak

Subject: [PATCH] posix_acl: Use try_cmpxchg in get_acl

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old
in get_acl. x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Alexander Viro <[email protected]>
---
fs/posix_acl.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index d7bc81fc0840..420c689e1bec 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -174,7 +174,7 @@ static struct posix_acl *__get_acl(struct user_namespace *mnt_userns,
* Cache the result, but only if our sentinel is still in place.
*/
posix_acl_dup(acl);
- if (unlikely(cmpxchg(p, sentinel, acl) != sentinel))
+ if (unlikely(!try_cmpxchg(p, &sentinel, acl)))
posix_acl_release(acl);
return acl;
}
--
2.38.1

2022-12-23 11:06:05

by Christian Brauner

Subject: Re: [PATCH] posix_acl: Use try_cmpxchg in get_acl

On Wed, Dec 21, 2022 at 08:35:40PM +0100, Uros Bizjak wrote:
> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old
> in get_acl. x86 CMPXCHG instruction returns success in ZF flag,
> so this change saves a compare after cmpxchg (and related move
> instruction in front of cmpxchg).
>
> No functional change intended.
>
> Signed-off-by: Uros Bizjak <[email protected]>
> Cc: Christian Brauner <[email protected]>
> Cc: Alexander Viro <[email protected]>
> ---

Looks ok to me. But it's not urgent so I'll circle back to this post
-rc1.

Thanks!
Christian

2023-01-08 12:18:15

by Christian Brauner

Subject: Re: [PATCH] posix_acl: Use try_cmpxchg in get_acl

On Fri, Dec 23, 2022 at 11:47:24AM +0100, Christian Brauner wrote:
> On Wed, Dec 21, 2022 at 08:35:40PM +0100, Uros Bizjak wrote:
> > Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old
> > in get_acl. x86 CMPXCHG instruction returns success in ZF flag,
> > so this change saves a compare after cmpxchg (and related move
> > instruction in front of cmpxchg).
> >
> > No functional change intended.
> >
> > Signed-off-by: Uros Bizjak <[email protected]>
> > Cc: Christian Brauner <[email protected]>
> > Cc: Alexander Viro <[email protected]>
> > ---
>
> Looks ok to me. But it's not urgent so I'll circle back to this post
> -rc1.

Seems good to me. So if there are no objectsions, then applied, thanks!

[1/1] posix_acl: Use try_cmpxchg in get_acl
commit: 4e1da8fe031303599e78f88e0dad9f44272e4f99