linux-kernel - [PATCH] io_uring: Use atomic_long_try_cmpxchg in __io_account

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Date:   Thu, 14 Jul 2022 18:33:01 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: [PATCH] io_uring: Use atomic_long_try_cmpxchg in __io_account_mem

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@...il.com>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Pavel Begunkov <asml.silence@...il.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