queue-6.19/rqspinlock-fix-tas-fallback-lock-entry-creation.patch

From c819a3a6f84735142bcd375e352d7926ee22fbe5 Mon Sep 17 00:00:00 2001
From: Sasha Levin <sashal@kernel.org>
Date: Thu, 22 Jan 2026 03:59:11 -0800
Subject: rqspinlock: Fix TAS fallback lock entry creation

From: Kumar Kartikeya Dwivedi <memxor@gmail.com>

[ Upstream commit 82f3b142c99cf44c7b1e70b7720169c646b9760f ]

The TAS fallback can be invoked directly when queued spin locks are
disabled, and through the slow path when paravirt is enabled for queued
spin locks. In the latter case, the res_spin_lock macro will attempt the
fast path and already hold the entry when entering the slow path. This
will lead to creation of extraneous entries that are not released, which
may cause false positives for deadlock detection.

Fix this by always preceding invocation of the TAS fallback in every
case with the grabbing of the held lock entry, and add a comment to make
note of this.

Fixes: c9102a68c070 ("rqspinlock: Add a test-and-set fallback")
Reported-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Tested-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260122115911.3668985-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/asm-generic/rqspinlock.h | 2 +-
 kernel/bpf/rqspinlock.c          | 7 ++++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/asm-generic/rqspinlock.h b/include/asm-generic/rqspinlock.h
index 0f2dcbbfee2f0..5c5cf2f7fc395 100644
--- a/include/asm-generic/rqspinlock.h
+++ b/include/asm-generic/rqspinlock.h
@@ -191,7 +191,7 @@ static __always_inline int res_spin_lock(rqspinlock_t *lock)
 
 #else
 
-#define res_spin_lock(lock) resilient_tas_spin_lock(lock)
+#define res_spin_lock(lock) ({ grab_held_lock_entry(lock); resilient_tas_spin_lock(lock); })
 
 #endif /* CONFIG_QUEUED_SPINLOCKS */
 
diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
index f7d0c8d4644ed..2fdfa828e3d35 100644
--- a/kernel/bpf/rqspinlock.c
+++ b/kernel/bpf/rqspinlock.c
@@ -265,10 +265,11 @@ int __lockfunc resilient_tas_spin_lock(rqspinlock_t *lock)
 
 	RES_INIT_TIMEOUT(ts);
 	/*
-	 * The fast path is not invoked for the TAS fallback, so we must grab
-	 * the deadlock detection entry here.
+	 * We are either called directly from res_spin_lock after grabbing the
+	 * deadlock detection entry when queued spinlocks are disabled, or from
+	 * resilient_queued_spin_lock_slowpath after grabbing the deadlock
+	 * detection entry. No need to obtain it here.
 	 */
-	grab_held_lock_entry(lock);
 
 	/*
 	 * Since the waiting loop's time is dependent on the amount of
-- 
2.51.0