[tip: locking/core] locking/semaphore: Remove the list_head from struct semaphore

["tip-bot2 for Matthew Wilcox (Oracle)" <tip-bot2@linutronix.de>]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     b9bdd4b6840454ef87f61b6506c9635c57a81650
Gitweb:        https://git.kernel.org/tip/b9bdd4b6840454ef87f61b6506c9635c57a81650
Author:        Matthew Wilcox (Oracle) <willy@infradead.org>
AuthorDate:    Thu, 05 Mar 2026 19:55:42 
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Sun, 08 Mar 2026 11:06:52 +01:00

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
---
 drivers/acpi/osl.c         |  2 +-
 include/linux/semaphore.h  |  4 ++--
 kernel/locking/semaphore.c | 41 +++++++++++++++++++++++++++----------
 3 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
index 5b77731..2af0db9 100644
--- a/drivers/acpi/osl.c
+++ b/drivers/acpi/osl.c
@@ -1257,7 +1257,7 @@ acpi_status acpi_os_delete_semaphore(acpi_handle handle)
 
 	ACPI_DEBUG_PRINT((ACPI_DB_MUTEX, "Deleting semaphore[%p].\n", handle));
 
-	BUG_ON(!list_empty(&sem->wait_list));
+	BUG_ON(sem->first_waiter);
 	kfree(sem);
 	sem = NULL;
 
diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
index 8970615..a4c8651 100644
--- a/include/linux/semaphore.h
+++ b/include/linux/semaphore.h
@@ -15,7 +15,7 @@
 struct semaphore {
 	raw_spinlock_t		lock;
 	unsigned int		count;
-	struct list_head	wait_list;
+	struct semaphore_waiter *first_waiter;
 
 #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
 	unsigned long		last_holder;
@@ -33,7 +33,7 @@ struct semaphore {
 {									\
 	.lock		= __RAW_SPIN_LOCK_UNLOCKED((name).lock),	\
 	.count		= n,						\
-	.wait_list	= LIST_HEAD_INIT((name).wait_list)		\
+	.first_waiter	= NULL						\
 	__LAST_HOLDER_SEMAPHORE_INITIALIZER				\
 }
 
diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
index 3ef032e..74d4143 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);
 }