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        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
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Subject: [PATCH v2 15/22] futex: Simplify double_lock_hb()
Date:   Thu, 23 Sep 2021 14:11:04 -0300
Message-Id: <20210923171111.300673-16-andrealmeid@collabora.com>
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From: Peter Zijlstra <peterz@infradead.org>

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.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@collabora.com>
[André: Add commit description]
Signed-off-by: André Almeida <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 fe847f57dad5..465f7bd5caef 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
-- 
2.33.0