Re: Regression on linux-next (next-20260324 )

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 21, 2026 at 10:56:48PM +0200, Peter Zijlstra wrote:
> Excellent, I'll write it up tomorrow.

How's this? It 'passes' the ww_mutex selftest thing in so far as that I
get the same:

[    2.312369] Beginning ww (wound) mutex selftests
[    4.853240] stress (stress_inorder_work) failed with -35
[    9.379572] Beginning ww (die) mutex selftests
[   16.435831] All ww mutex selftests passed

before the offending commit and after this patch.

---
Subject: Subject: locking/mutex: Fix ww_mutex wait_list operations
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Apr 22 10:38:41 CEST 2026

Chaitanya and John reported commit 25500ba7e77c ("locking/mutex: Remove the
list_head from struct mutex") wrecked ww_mutex.

Specifically there were 2 issues:

 - __ww_waiter_prev() had the termination condition wrong; it would terminate
   when the previous entry was the first, which results in a truncated
   iteration: W3, W2, (no W1).

 - __mutex_add_waiter(@pos != NULL), as used by __ww_waiter_add() /
   __ww_mutex_add_waiter(); this inserts @waiter before @pos (which is what
   list_add_tail() does). But this should then also update lock->first_waiter.

Much thanks to Prateek for spotting the __mutex_add_waiter() issue!

Fixes: 25500ba7e77c ("locking/mutex: Remove the list_head from struct mutex")
Reported-by: "Borah, Chaitanya Kumar" <chaitanya.kumar.borah@intel.com>
Closes: https://lore.kernel.org/r/af005996-05e9-4336-8450-d14ca652ba5d%40intel.com
Reported-by: John Stultz <jstultz@google.com>
Closes: https://lore.kernel.org/r/CANDhNCq%3Doizzud3hH3oqGzTrcjB8OwGeineJ3mwZuGdDWG8fRQ%40mail.gmail.com
Debugged-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/locking/mutex.c    |   40 +++++++++++++++++++++++++++-------------
 kernel/locking/ww_mutex.h |   34 ++++++++++++++++++++++++++++++++--
 2 files changed, 59 insertions(+), 15 deletions(-)

--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -198,27 +198,43 @@ static inline void __mutex_clear_flag(st
 }
 
 /*
- * Add @waiter to a given location in the lock wait_list and set the
- * FLAG_WAITERS flag if it's the first waiter.
+ * Add @waiter to the @lock wait_list and set the FLAG_WAITERS flag if it's
+ * the first waiter.
+ *
+ * When @pos, @waiter is added before the waiter indicated by @pos. Otherwise
+ * @waiter will be added to the tail of the list.
  */
 static void
 __mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
-		   struct mutex_waiter *first)
+		   struct mutex_waiter *pos)
 	__must_hold(&lock->wait_lock)
 {
+	struct mutex_waiter *first = lock->first_waiter;
+
 	hung_task_set_blocker(lock, BLOCKER_TYPE_MUTEX);
 	debug_mutex_add_waiter(lock, waiter, current);
 
-	if (!first)
-		first = lock->first_waiter;
+	if (pos) {
+		/*
+		 * Insert @waiter before @pos.
+		 */
+		list_add_tail(&waiter->list, &pos->list);
+		/*
+		 * If @pos == @first, then @waiter will be the new first.
+		 */
+		if (pos == first)
+			lock->first_waiter = waiter;
+		return;
+	}
 
 	if (first) {
 		list_add_tail(&waiter->list, &first->list);
-	} else {
-		INIT_LIST_HEAD(&waiter->list);
-		lock->first_waiter = waiter;
-		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
+		return;
 	}
+
+	INIT_LIST_HEAD(&waiter->list);
+	lock->first_waiter = waiter;
+	__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
 }
 
 static void
@@ -229,10 +245,8 @@ __mutex_remove_waiter(struct mutex *lock
 		__mutex_clear_flag(lock, MUTEX_FLAGS);
 		lock->first_waiter = NULL;
 	} else {
-		if (lock->first_waiter == waiter) {
-			lock->first_waiter = list_first_entry(&waiter->list,
-							      struct mutex_waiter, list);
-		}
+		if (lock->first_waiter == waiter)
+			lock->first_waiter = list_next_entry(waiter, list);
 		list_del(&waiter->list);
 	}
 
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -6,6 +6,19 @@
 #define MUTEX_WAITER	mutex_waiter
 #define WAIT_LOCK	wait_lock
 
+/*
+ *           +--------+
+ *           | first  |
+ *           +--------+
+ *                |
+ *                v
+ *  +----+     +----+     +----+
+ *  | W3 | <-> | W1 | <-> | W2 |
+ *  +----+     +----+     +----+
+ *    ^                     ^
+ *    +---------------------+
+ */
+
 static inline struct mutex_waiter *
 __ww_waiter_first(struct mutex *lock)
 	__must_hold(&lock->wait_lock)
@@ -13,26 +26,43 @@ __ww_waiter_first(struct mutex *lock)
 	return lock->first_waiter;
 }
 
+/*
+ * for (cur = __ww_waiter_first(); cur; cur = __ww_waiter_next())
+ *
+ * Should iterate like: W1, W2, W3
+ */
 static inline struct mutex_waiter *
 __ww_waiter_next(struct mutex *lock, struct mutex_waiter *w)
 	__must_hold(&lock->wait_lock)
 {
 	w = list_next_entry(w, list);
+	/*
+	 * Terminate if the next entry is the first again, that has already
+	 * been observed.
+	 */
 	if (lock->first_waiter == w)
 		return NULL;
 
 	return w;
 }
 
+/*
+ * for (cur = __ww_waiter_last(); cur; cur = __ww_waiter_prev())
+ *
+ * Should iterate like: W3, W2, W1
+ */
 static inline struct mutex_waiter *
 __ww_waiter_prev(struct mutex *lock, struct mutex_waiter *w)
 	__must_hold(&lock->wait_lock)
 {
-	w = list_prev_entry(w, list);
+	/*
+	 * Terminate at the first entry, the previous entry of first is the
+	 * last and that has already been observed.
+	 */
 	if (lock->first_waiter == w)
 		return NULL;
 
-	return w;
+	return list_prev_entry(w, list);
 }
 
 static inline struct mutex_waiter *