locking/mutex: Remove the list_head from struct mutex

Instead of embedding a list_head in struct mutex, store a pointer to
the first waiter.  The list of waiters remains a doubly linked list so
we can efficiently add to the tail of the list, remove from the front
(or middle) of the list.

Some of the list manipulation becomes more complicated, but it's a
reasonable tradeoff on the slow paths to shrink data structures which
embed a mutex like struct file.

Some of the debug checks have to be deleted because there's no equivalent
to checking them in the new scheme (eg an empty waiter->list now means
that it is the only waiter, not that the waiter is no longer on the list).

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260305195545.3707590-4-willy@infradead.org

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 2f648ee204e7c5124a8af664faed0236edbdbc67..c471b129f70399b348fcedfb2dd4ad19a66462f1 100644 (file)
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -79,7 +79,7 @@ do {                                                                  \
 #define __MUTEX_INITIALIZER(lockname) \
                { .owner = ATOMIC_LONG_INIT(0) \
                , .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(lockname.wait_lock) \
-               , .wait_list = LIST_HEAD_INIT(lockname.wait_list) \
+               , .first_waiter = NULL \
                __DEBUG_MUTEX_INITIALIZER(lockname) \
                __DEP_MAP_MUTEX_INITIALIZER(lockname) }
 

diff --git a/include/linux/mutex_types.h b/include/linux/mutex_types.h
index 80975935ec4887b016c22dd909a4db79bb16d93f..a8f119f81177da029df29611d5170a66da982290 100644 (file)
--- a/include/linux/mutex_types.h
+++ b/include/linux/mutex_types.h
@@ -44,7 +44,7 @@ context_lock_struct(mutex) {
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
        struct optimistic_spin_queue osq; /* Spinner MCS lock */
 #endif
-       struct list_head        wait_list;
+       struct mutex_waiter     *first_waiter;
 #ifdef CONFIG_DEBUG_MUTEXES
        void                    *magic;
 #endif

diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 2c6b02d4699be705ec85526c8b0826153342bac1..94930d506bcfda83f2dd6ab8d559324cfb4c3f0f 100644 (file)
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -37,9 +37,8 @@ void debug_mutex_lock_common(struct mutex *lock, struct mutex_waiter *waiter)
 void debug_mutex_wake_waiter(struct mutex *lock, struct mutex_waiter *waiter)
 {
        lockdep_assert_held(&lock->wait_lock);
-       DEBUG_LOCKS_WARN_ON(list_empty(&lock->wait_list));
+       DEBUG_LOCKS_WARN_ON(!lock->first_waiter);
        DEBUG_LOCKS_WARN_ON(waiter->magic != waiter);
-       DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
 }
 
 void debug_mutex_free_waiter(struct mutex_waiter *waiter)
@@ -62,7 +61,6 @@ void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 {
        struct mutex *blocked_on = __get_task_blocked_on(task);
 
-       DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
        DEBUG_LOCKS_WARN_ON(waiter->task != task);
        DEBUG_LOCKS_WARN_ON(blocked_on && blocked_on != lock);
 
@@ -74,7 +72,6 @@ void debug_mutex_unlock(struct mutex *lock)
 {
        if (likely(debug_locks)) {
                DEBUG_LOCKS_WARN_ON(lock->magic != lock);
-               DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
        }
 }
 

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index c867f6c15530d59354c643502e3ca8946d203e5c..95f1822122a1c9401102b5f7ac8f87f603bda2f4 100644 (file)
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -47,7 +47,7 @@ static void __mutex_init_generic(struct mutex *lock)
 {
        atomic_long_set(&lock->owner, 0);
        raw_spin_lock_init(&lock->wait_lock);
-       INIT_LIST_HEAD(&lock->wait_list);
+       lock->first_waiter = NULL;
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
        osq_lock_init(&lock->osq);
 #endif
@@ -194,33 +194,42 @@ static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag)
        atomic_long_andnot(flag, &lock->owner);
 }
 
-static inline bool __mutex_waiter_is_first(struct mutex *lock, struct mutex_waiter *waiter)
-{
-       return list_first_entry(&lock->wait_list, struct mutex_waiter, list) == waiter;
-}
-
 /*
  * Add @waiter to a given location in the lock wait_list and set the
  * FLAG_WAITERS flag if it's the first waiter.
  */
 static void
 __mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
-                  struct list_head *list)
+                  struct mutex_waiter *first)
 {
        hung_task_set_blocker(lock, BLOCKER_TYPE_MUTEX);
        debug_mutex_add_waiter(lock, waiter, current);
 
-       list_add_tail(&waiter->list, list);
-       if (__mutex_waiter_is_first(lock, waiter))
+       if (!first)
+               first = lock->first_waiter;
+
+       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);
+       }
 }
 
 static void
 __mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter)
 {
-       list_del(&waiter->list);
-       if (likely(list_empty(&lock->wait_list)))
+       if (list_empty(&waiter->list)) {
                __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);
+               }
+               list_del(&waiter->list);
+       }
 
        debug_mutex_remove_waiter(lock, waiter, current);
        hung_task_clear_blocker();
@@ -340,7 +349,7 @@ bool ww_mutex_spin_on_owner(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
         * Similarly, stop spinning if we are no longer the
         * first waiter.
         */
-       if (waiter && !__mutex_waiter_is_first(lock, waiter))
+       if (waiter && lock->first_waiter != waiter)
                return false;
 
        return true;
@@ -645,7 +654,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
        if (!use_ww_ctx) {
                /* add waiting tasks to the end of the waitqueue (FIFO): */
-               __mutex_add_waiter(lock, &waiter, &lock->wait_list);
+               __mutex_add_waiter(lock, &waiter, NULL);
        } else {
                /*
                 * Add in stamp order, waking up waiters that must kill
@@ -691,7 +700,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
                schedule_preempt_disabled();
 
-               first = __mutex_waiter_is_first(lock, &waiter);
+               first = lock->first_waiter == &waiter;
 
                /*
                 * As we likely have been woken up by task
@@ -734,8 +743,7 @@ acquired:
                 * Wound-Wait; we stole the lock (!first_waiter), check the
                 * waiters as anyone might want to wound us.
                 */
-               if (!ww_ctx->is_wait_die &&
-                   !__mutex_waiter_is_first(lock, &waiter))
+               if (!ww_ctx->is_wait_die && lock->first_waiter != &waiter)
                        __ww_mutex_check_waiters(lock, ww_ctx, &wake_q);
        }
 
@@ -931,6 +939,7 @@ EXPORT_SYMBOL_GPL(ww_mutex_lock_interruptible);
 static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)
 {
        struct task_struct *next = NULL;
+       struct mutex_waiter *waiter;
        DEFINE_WAKE_Q(wake_q);
        unsigned long owner;
        unsigned long flags;
@@ -962,12 +971,8 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 
        raw_spin_lock_irqsave(&lock->wait_lock, flags);
        debug_mutex_unlock(lock);
-       if (!list_empty(&lock->wait_list)) {
-               /* get the first entry from the wait-list: */
-               struct mutex_waiter *waiter =
-                       list_first_entry(&lock->wait_list,
-                                        struct mutex_waiter, list);
-
+       waiter = lock->first_waiter;
+       if (waiter) {
                next = waiter->task;
 
                debug_mutex_wake_waiter(lock, waiter);

diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 31a785afee6c0ebfa36a2ceecc76e8fd27c9a739..a0847e91ae0417a42537d102082b2cf045960e8d 100644 (file)
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -8,20 +8,14 @@
 static inline struct mutex_waiter *
 __ww_waiter_first(struct mutex *lock)
 {
-       struct mutex_waiter *w;
-
-       w = list_first_entry(&lock->wait_list, struct mutex_waiter, list);
-       if (list_entry_is_head(w, &lock->wait_list, list))
-               return NULL;
-
-       return w;
+       return lock->first_waiter;
 }
 
 static inline struct mutex_waiter *
 __ww_waiter_next(struct mutex *lock, struct mutex_waiter *w)
 {
        w = list_next_entry(w, list);
-       if (list_entry_is_head(w, &lock->wait_list, list))
+       if (lock->first_waiter == w)
                return NULL;
 
        return w;
@@ -31,7 +25,7 @@ static inline struct mutex_waiter *
 __ww_waiter_prev(struct mutex *lock, struct mutex_waiter *w)
 {
        w = list_prev_entry(w, list);
-       if (list_entry_is_head(w, &lock->wait_list, list))
+       if (lock->first_waiter == w)
                return NULL;
 
        return w;
@@ -40,22 +34,17 @@ __ww_waiter_prev(struct mutex *lock, struct mutex_waiter *w)
 static inline struct mutex_waiter *
 __ww_waiter_last(struct mutex *lock)
 {
-       struct mutex_waiter *w;
-
-       w = list_last_entry(&lock->wait_list, struct mutex_waiter, list);
-       if (list_entry_is_head(w, &lock->wait_list, list))
-               return NULL;
+       struct mutex_waiter *w = lock->first_waiter;
 
+       if (w)
+               w = list_prev_entry(w, list);
        return w;
 }
 
 static inline void
 __ww_waiter_add(struct mutex *lock, struct mutex_waiter *waiter, struct mutex_waiter *pos)
 {
-       struct list_head *p = &lock->wait_list;
-       if (pos)
-               p = &pos->list;
-       __mutex_add_waiter(lock, waiter, p);
+       __mutex_add_waiter(lock, waiter, pos);
 }
 
 static inline struct task_struct *