From: Peter Zijlstra Date: Wed, 22 Apr 2026 08:38:41 +0000 (+0200) Subject: locking/mutex: Fix ww_mutex wait_list operations X-Git-Tag: v7.1-rc1~25^2 X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=0adc92b910b3d6bf4913d79869365d553154a070;p=thirdparty%2Fkernel%2Flinux.git 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" Closes: https://lore.kernel.org/r/af005996-05e9-4336-8450-d14ca652ba5d%40intel.com Reported-by: John Stultz Closes: https://lore.kernel.org/r/CANDhNCq%3Doizzud3hH3oqGzTrcjB8OwGeineJ3mwZuGdDWG8fRQ%40mail.gmail.com Reported-by: Mikhail Gavrilov Closes: https://lore.kernel.org/r/CABXGCsO5fKq2nD9nO8yO1z50ZzgCPWqueNXHANjntaswoOh2Dg@mail.gmail.com Debugged-by: K Prateek Nayak Signed-off-by: Peter Zijlstra (Intel) Tested-by: K Prateek Nayak Tested-by: Mikhail Gavrilov Link: https://patch.msgid.link/20260422092335.GH3102924%40noisy.programming.kicks-ass.net --- diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 186b463fe326..09534628dc01 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 016f0db892a5..6c12452097e1 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 *