[tip: locking/urgent] locking/mutex: Fix ww_mutex wait_list operations
From: tip-bot2 for Peter Zijlstra
Date: Thu Apr 23 2026 - 04:10:01 EST
The following commit has been merged into the locking/urgent branch of tip:
Commit-ID: 0adc92b910b3d6bf4913d79869365d553154a070
Gitweb: https://git.kernel.org/tip/0adc92b910b3d6bf4913d79869365d553154a070
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Wed, 22 Apr 2026 10:38:41 +02:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 23 Apr 2026 10:05:49 +02:00
locking/mutex: Fix ww_mutex wait_list operations
Chaitanya, John and Mikhail 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@xxxxxxxxx>
Closes: https://lore.kernel.org/r/af005996-05e9-4336-8450-d14ca652ba5d%40intel.com
Reported-by: John Stultz <jstultz@xxxxxxxxxx>
Closes: https://lore.kernel.org/r/CANDhNCq%3Doizzud3hH3oqGzTrcjB8OwGeineJ3mwZuGdDWG8fRQ%40mail.gmail.com
Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
Closes: https://lore.kernel.org/r/CABXGCsO5fKq2nD9nO8yO1z50ZzgCPWqueNXHANjntaswoOh2Dg@xxxxxxxxxxxxxx
Debugged-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
Link: https://patch.msgid.link/20260422092335.GH3102924%40noisy.programming.kicks-ass.net
---
kernel/locking/mutex.c | 40 +++++++++++++++++++++++++-------------
kernel/locking/ww_mutex.h | 34 ++++++++++++++++++++++++++++++--
2 files changed, 59 insertions(+), 15 deletions(-)
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 186b463..0953462 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -198,27 +198,43 @@ static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag)
}
/*
- * 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, 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 016f0db..6c12452 100644
--- 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 *