linux-kernel - [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to reduce false positives

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Message-ID: <20250612042005.99602-3-lance.yang@linux.dev>
Date: Thu, 12 Jun 2025 12:19:25 +0800
From: Lance Yang <ioworker0@...il.com>
To: akpm@...ux-foundation.org
Cc: zi.li@...ux.dev,
	anna.schumaker@...cle.com,
	boqun.feng@...il.com,
	joel.granados@...nel.org,
	jstultz@...gle.com,
	kent.overstreet@...ux.dev,
	leonylgao@...cent.com,
	linux-kernel@...r.kernel.org,
	longman@...hat.com,
	mhiramat@...nel.org,
	mingo@...hat.com,
	mingzhe.yang@...com,
	peterz@...radead.org,
	rostedt@...dmis.org,
	senozhatsky@...omium.org,
	tfiga@...omium.org,
	will@...nel.org,
	Lance Yang <lance.yang@...ux.dev>
Subject: [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to reduce false positives

From: Lance Yang <lance.yang@...ux.dev>

When CONFIG_DETECT_HUNG_TASK_BLOCKER is enabled, a stale owner pointer in a
reader-owned rwsem can lead to false positives in blocker tracking.

To mitigate this, let’s try to clear the owner field on unlock, as a NULL
owner is better than a stale one for diagnostics.

Signed-off-by: Lance Yang <lance.yang@...ux.dev>
---
 kernel/locking/rwsem.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 6cb29442d4fc..a310eb9896de 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -205,14 +205,12 @@ bool is_rwsem_reader_owned(struct rw_semaphore *sem)
 		return false;
 	return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
 }
-#endif
 
-#ifdef CONFIG_DEBUG_RWSEMS
 /*
- * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
- * is a task pointer in owner of a reader-owned rwsem, it will be the
- * real owner or one of the real owners. The only exception is when the
- * unlock is done by up_read_non_owner().
+ * With CONFIG_DEBUG_RWSEMS or CONFIG_DETECT_HUNG_TASK_BLOCKER configured,
+ * it will make sure that the owner field of a reader-owned rwsem either
+ * points to a real reader-owner(s) or gets cleared. The only exception is
+ * when the unlock is done by up_read_non_owner().
  */
 static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
 {
-- 
2.49.0