linux-kernel - [PATCH v2] futex: Rewrite get_inode_sequence

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Message-ID: <20241004085257.10908-1-ubizjak@gmail.com>
Date: Fri,  4 Oct 2024 10:52:32 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	André Almeida <andrealmeid@...lia.com>
Subject: [PATCH v2] futex: Rewrite get_inode_sequence_number() to make code simpler

Rewrite get_inode_sequence_number() to make code simpler:

a) Rewrite FOR loop to a DO-WHILE loop with returns moved
out of the loop.

b) Use atomic64_inc_return() instead of atomic64_add_return().

c) Use !atomic64_try_cmpxchg_relaxed(*ptr, &old, new) instead of
atomic64_cmpxchg_relaxed (*ptr, old, new) != old.  x86 CMPXCHG
instruction returns success in ZF flag, so this change also saves
a compare instruction after CMPXCHG.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Darren Hart <dvhart@...radead.org>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: "André Almeida" <andrealmeid@...lia.com>
---
v2: Explicitly initialize "old" to zero before the call to
atomic64_try_cmpxchg_relaxed(). Rewrite commit message to
state the motivation for the patch.
---
 kernel/futex/core.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 136768ae2637..ac650f7ed56c 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -173,23 +173,21 @@ futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
 static u64 get_inode_sequence_number(struct inode *inode)
 {
 	static atomic64_t i_seq;
-	u64 old;
+	u64 old, new;
 
 	/* Does the inode already have a sequence number? */
 	old = atomic64_read(&inode->i_sequence);
 	if (likely(old))
 		return old;
 
-	for (;;) {
-		u64 new = atomic64_add_return(1, &i_seq);
-		if (WARN_ON_ONCE(!new))
-			continue;
+	do {
+		new = atomic64_inc_return(&i_seq);
+	} while	(WARN_ON_ONCE(!new));
 
-		old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
-		if (old)
-			return old;
-		return new;
-	}
+	old = 0;
+	if (!atomic64_try_cmpxchg_relaxed(&inode->i_sequence, &old, new))
+		return old;
+	return new;
 }
 
 /**
-- 
2.46.2