linux-kernel - [PATCH] posix-timers: use atomic64_try_cmpxchg in __update_gt

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Date:   Mon, 16 Jan 2023 17:53:37 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] posix-timers: use atomic64_try_cmpxchg in __update_gt_cputime

Use atomic64_try_cmpxchg instead of atomic64_cmpxchg (*ptr, old, new) == old
in __update_gt_cputime.  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, atomic64_try_cmpxchg implicitly assigns old *ptr value to "old" when
cmpxchg fails.  There is no need to re-read the value in the loop.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/posix-cpu-timers.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index cb925e8ef9a8..2f5e9b34022c 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -243,13 +243,12 @@ static void proc_sample_cputime_atomic(struct task_cputime_atomic *at,
  */
 static inline void __update_gt_cputime(atomic64_t *cputime, u64 sum_cputime)
 {
-	u64 curr_cputime;
-retry:
-	curr_cputime = atomic64_read(cputime);
-	if (sum_cputime > curr_cputime) {
-		if (atomic64_cmpxchg(cputime, curr_cputime, sum_cputime) != curr_cputime)
-			goto retry;
-	}
+	u64 curr_cputime = atomic64_read(cputime);
+
+	do {
+		if (sum_cputime <= curr_cputime)
+			return;
+	} while (!atomic64_try_cmpxchg(cputime, &curr_cputime, sum_cputime));
 }
 
 static void update_gt_cputime(struct task_cputime_atomic *cputime_atomic,
-- 
2.39.0