[PATCH] io_uring: Use atomic_long_try_cmpxchg in __io_account_mem

[PATCH] io_uring: Use atomic_long_try_cmpxchg in __io_account_mem

[Uros Bizjak]

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

Also, atomic_long_try_cmpxchg implicitly assigns old *ptr value
to "old" when cmpxchg fails, enabling further code simplifications.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index a01ea49f3017..9f82904dcdae 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -10504,14 +10504,13 @@ static inline int __io_account_mem(struct user_struct *user,
 	/* Don't allow more pages than we can safely lock */
 	page_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
 
+	cur_pages = atomic_long_read(&user->locked_vm);
 	do {
-		cur_pages = atomic_long_read(&user->locked_vm);
 		new_pages = cur_pages + nr_pages;
 		if (new_pages > page_limit)
 			return -ENOMEM;
-	} while (atomic_long_cmpxchg(&user->locked_vm, cur_pages,
-					new_pages) != cur_pages);
-
+	} while (!atomic_long_try_cmpxchg(&user->locked_vm,
+					  &cur_pages, new_pages));
 	return 0;
 }
 
-- 
2.35.3

Re: [PATCH] io_uring: Use atomic_long_try_cmpxchg in __io_account_mem

[Jens Axboe]

On 7/14/22 10:33 AM, Uros Bizjak wrote:
> Use atomic_long_try_cmpxchg instead of
> atomic_long_cmpxchg (*ptr, old, new) == old in __io_account_mem.
> x86 CMPXCHG instruction returns success in ZF flag, so this
> change saves a compare after cmpxchg (and related move
> instruction in front of cmpxchg).
> 
> Also, atomic_long_try_cmpxchg implicitly assigns old *ptr value
> to "old" when cmpxchg fails, enabling further code simplifications.
> 
> No functional change intended.

This will be io_uring/rsrc.c for the for-next branches, but it'll apply
directly as that with a slight offset:

checking file io_uring/rsrc.c
Hunk #1 succeeded at 56 (offset -10448 lines).

I'll do that, thanks.

-- 
Jens Axboe