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) <willy@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260305195545.3707590-3-willy@infradead.org

1 files changed, 31 insertions, 10 deletions

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 <linux/compiler.h>
@@ -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);
 }