linux-kernel - [PATCH] aio: Use atomic_try_cmpxchg in __get_reqs

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Message-Id: <20220714164851.3055-1-ubizjak@gmail.com>
Date:   Thu, 14 Jul 2022 18:48:51 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Benjamin LaHaise <bcrl@...ck.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: [PATCH] aio: Use atomic_try_cmpxchg in __get_reqs_available

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in __get_reqs_available. 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_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: Benjamin LaHaise <bcrl@...ck.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
---
 fs/aio.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 3c249b938632..054897f59c5e 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -951,16 +951,13 @@ static bool __get_reqs_available(struct kioctx *ctx)
 	local_irq_save(flags);
 	kcpu = this_cpu_ptr(ctx->cpu);
 	if (!kcpu->reqs_available) {
-		int old, avail = atomic_read(&ctx->reqs_available);
+		int avail = atomic_read(&ctx->reqs_available);
 
 		do {
 			if (avail < ctx->req_batch)
 				goto out;
-
-			old = avail;
-			avail = atomic_cmpxchg(&ctx->reqs_available,
-					       avail, avail - ctx->req_batch);
-		} while (avail != old);
+		} while (!atomic_try_cmpxchg(&ctx->reqs_available,
+					     &avail, avail - ctx->req_batch));
 
 		kcpu->reqs_available += ctx->req_batch;
 	}
-- 
2.35.3