linux-kernel - [tip: locking/core] futex: Simplify double_lock

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Message-ID: <163377402559.25758.4094406809962751599.tip-bot2@tip-bot2>
Date:   Sat, 09 Oct 2021 10:07:05 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        andrealmeid@...labora.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] futex: Simplify double_lock_hb()

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     bff7c57c2f50dca7b5e320f499e0898c3fb8a9ff
Gitweb:        https://git.kernel.org/tip/bff7c57c2f50dca7b5e320f499e0898c3fb8a9ff
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Thu, 23 Sep 2021 14:11:04 -03:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 07 Oct 2021 13:51:11 +02:00

futex: Simplify double_lock_hb()

We need to make sure that all requeue operations take the hash bucket
locks in the same order to avoid deadlock. Simplify the current
double_lock_hb implementation by making sure hb1 is always the
"smallest" bucket to avoid extra checks.

[André: Add commit description]
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: André Almeida <andrealmeid@...labora.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: André Almeida <andrealmeid@...labora.com>
Link: https://lore.kernel.org/r/20210923171111.300673-16-andrealmeid@collabora.com
---
 kernel/futex/futex.h | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index fe847f5..465f7bd 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -239,14 +239,12 @@ extern int fixup_pi_owner(u32 __user *uaddr, struct futex_q *q, int locked);
 static inline void
 double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
 {
-	if (hb1 <= hb2) {
-		spin_lock(&hb1->lock);
-		if (hb1 < hb2)
-			spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
-	} else { /* hb1 > hb2 */
-		spin_lock(&hb2->lock);
-		spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
-	}
+	if (hb1 > hb2)
+		swap(hb1, hb2);
+
+	spin_lock(&hb1->lock);
+	if (hb1 != hb2)
+		spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
 }
 
 static inline void