From b9bdd4b6840454ef87f61b6506c9635c57a81650 Mon Sep 17 00:00:00 2001 From: "Matthew Wilcox (Oracle)" Date: Thu, 5 Mar 2026 19:55:42 +0000 Subject: locking/semaphore: Remove the list_head from struct semaphore Instead of embedding a list_head in struct semaphore, 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 and 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 semaphore. Signed-off-by: Matthew Wilcox (Oracle) Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260305195545.3707590-3-willy@infradead.org --- kernel/locking/semaphore.c | 41 +++++++++++++++++++++++++++++++---------- 1 file changed, 31 insertions(+), 10 deletions(-) (limited to 'kernel/locking') diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c index 3ef032e22f7e..74d41433ba13 100644 --- a/kernel/locking/semaphore.c +++ b/kernel/locking/semaphore.c @@ -21,7 +21,7 @@ * too. * * The ->count variable represents how many more tasks can acquire this - * semaphore. If it's zero, there may be tasks waiting on the wait_list. + * semaphore. If it's zero, there may be waiters. */ #include @@ -226,7 +226,7 @@ void __sched up(struct semaphore *sem) hung_task_sem_clear_if_holder(sem); - if (likely(list_empty(&sem->wait_list))) + if (likely(!sem->first_waiter)) sem->count++; else __up(sem, &wake_q); @@ -244,6 +244,21 @@ struct semaphore_waiter { bool up; }; +static inline +void sem_del_waiter(struct semaphore *sem, struct semaphore_waiter *waiter) +{ + if (list_empty(&waiter->list)) { + sem->first_waiter = NULL; + return; + } + + if (sem->first_waiter == waiter) { + sem->first_waiter = list_first_entry(&waiter->list, + struct semaphore_waiter, list); + } + list_del(&waiter->list); +} + /* * Because this function is inlined, the 'state' parameter will be * constant, and thus optimised away by the compiler. Likewise the @@ -252,9 +267,15 @@ struct semaphore_waiter { static inline int __sched ___down_common(struct semaphore *sem, long state, long timeout) { - struct semaphore_waiter waiter; - - list_add_tail(&waiter.list, &sem->wait_list); + struct semaphore_waiter waiter, *first; + + first = sem->first_waiter; + if (first) { + list_add_tail(&waiter.list, &first->list); + } else { + INIT_LIST_HEAD(&waiter.list); + sem->first_waiter = &waiter; + } waiter.task = current; waiter.up = false; @@ -274,11 +295,11 @@ static inline int __sched ___down_common(struct semaphore *sem, long state, } timed_out: - list_del(&waiter.list); + sem_del_waiter(sem, &waiter); return -ETIME; interrupted: - list_del(&waiter.list); + sem_del_waiter(sem, &waiter); return -EINTR; } @@ -321,9 +342,9 @@ static noinline int __sched __down_timeout(struct semaphore *sem, long timeout) static noinline void __sched __up(struct semaphore *sem, struct wake_q_head *wake_q) { - struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list, - struct semaphore_waiter, list); - list_del(&waiter->list); + struct semaphore_waiter *waiter = sem->first_waiter; + + sem_del_waiter(sem, waiter); waiter->up = true; wake_q_add(wake_q, waiter->task); } -- cgit v1.2.3