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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Mar 30, 2026 at 09:50:37PM +0200, Peter Zijlstra wrote:
> On Mon, Mar 30, 2026 at 01:56:33PM +0530, Borah, Chaitanya Kumar wrote:
> > > diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
> > > index b1834ab7e782..bb8b410779d4 100644
> > > --- a/kernel/locking/ww_mutex.h
> > > +++ b/kernel/locking/ww_mutex.h
> > > @@ -42,7 +42,7 @@ __ww_waiter_last(struct mutex *lock)
> > >   	struct mutex_waiter *w = lock->first_waiter;
> > >   	if (w)
> > > -		w = list_prev_entry(w, list);
> > > +		w = __ww_waiter_prev(lock, w);
> > >   	return w;
> > >   }
> > Thank you for the response, Peter. Unfortunately, the issue is still seen
> > with this change.
> 
> Bah, indeed. Looking at this after the weekend I see that it's actually
> wrong.
> 
> But I haven't yet had a new idea. I don't suppose there is a relatively
> easy way to reproduce this issue outside of your CI robot?
> 
> My current working thesis is that since this is graphics, this is
> ww_mutex related. I'll go over this code once more...

Since you've not provided a reproducer, can I ask you to try the below?

---
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 186b463fe326..a93e57fc53b1 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -229,10 +229,8 @@ __mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter)
 		__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);
 	}
 
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 016f0db892a5..875b303511b3 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -6,6 +6,32 @@
 #define MUTEX_WAITER	mutex_waiter
 #define WAIT_LOCK	wait_lock
 
+/*
+ *                +-------+
+ *                |   3   | <+
+ *                +-------+  |
+ *                    ^      |
+ *                    |      |
+ *                    v      |
+ *  +-------+     +-------+  |
+ *  | first | --> |   1   |  |
+ *  +-------+     +-------+  |
+ *                    ^      |
+ *                    |      |
+ *                    v      |
+ *                +-------+  |
+ *                |   2   | <+
+ *                +-------+
+ */
+
+/*
+ * Specifically:
+ *
+ *   for (cur = __ww_waiter_first(); cur; cur = __ww_waiter_next())
+ *     ...
+ *
+ * should iterate like: 1 2 3
+ */
 static inline struct mutex_waiter *
 __ww_waiter_first(struct mutex *lock)
 	__must_hold(&lock->wait_lock)
@@ -18,23 +44,21 @@ __ww_waiter_next(struct mutex *lock, struct mutex_waiter *w)
 	__must_hold(&lock->wait_lock)
 {
 	w = list_next_entry(w, list);
-	if (lock->first_waiter == w)
-		return NULL;
-
-	return w;
-}
-
-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);
-	if (lock->first_waiter == w)
+	/* We've already seen first, terminate */
+	if (w == __ww_waiter_first(lock))
 		return NULL;
 
 	return w;
 }
 
+/*
+ * Specifically:
+ *
+ *   for (cur = __ww_waiter_last(); cur; cur = __ww_waiter_prev())
+ *     ...
+ *
+ * should iterate like: 3 2 1
+ */
 static inline struct mutex_waiter *
 __ww_waiter_last(struct mutex *lock)
 	__must_hold(&lock->wait_lock)
@@ -46,6 +70,18 @@ __ww_waiter_last(struct mutex *lock)
 	return w;
 }
 
+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);
+	/* We've already seen last, terminate */
+	if (w == __ww_waiter_last(lock))
+		return NULL;
+
+	return w;
+}
+
 static inline void
 __ww_waiter_add(struct mutex *lock, struct mutex_waiter *waiter, struct mutex_waiter *pos)
 	__must_hold(&lock->wait_lock)