Re: [PATCH 0/3] Shrink size of sleeping locks

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 05, 2026 at 07:55:40PM +0000, Matthew Wilcox (Oracle) wrote:

> Matthew Wilcox (Oracle) (3):
>   rwsem: Remove the list_head from struct rw_semaphore
>   semaphore: Remove the list_head from struct semaphore
>   mutex: Remove the list_head from struct mutex
> 
>  drivers/acpi/osl.c           |  2 +-
>  include/linux/mutex.h        |  2 +-
>  include/linux/mutex_types.h  |  2 +-
>  include/linux/rwsem.h        |  8 ++--
>  include/linux/semaphore.h    |  4 +-
>  kernel/locking/mutex-debug.c |  5 +-
>  kernel/locking/mutex.c       | 49 +++++++++++---------
>  kernel/locking/rwsem.c       | 89 +++++++++++++++++++++++-------------
>  kernel/locking/semaphore.c   | 41 +++++++++++++----
>  kernel/locking/ww_mutex.h    | 25 +++-------
>  10 files changed, 132 insertions(+), 95 deletions(-)

Right, it also completely messes up my context analysis patches, but
that's my own damn fault for not having merged them yet, so I rebased
them on top of this.

I've also added a patch for rwsem since I was there anyway.

I shall push out the entire pile into queue/locking/core for the robots
to chew on.

Thanks!

---
Subject: locking/rwsem: Add context analysis
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri Mar 6 10:43:56 CET 2026

Add compiler context analysis annotations.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/rwsem.h      |    4 ++--
 kernel/locking/Makefile    |    1 +
 kernel/locking/rwbase_rt.c |    1 +
 kernel/locking/rwsem.c     |   25 +++++++++++++++++++++++--
 4 files changed, 27 insertions(+), 4 deletions(-)

--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -57,7 +57,7 @@ context_lock_struct(rw_semaphore) {
 	struct optimistic_spin_queue osq; /* spinner MCS lock */
 #endif
 	raw_spinlock_t wait_lock;
-	struct rwsem_waiter *first_waiter;
+	struct rwsem_waiter *first_waiter __guarded_by(&wait_lock);
 #ifdef CONFIG_DEBUG_RWSEMS
 	void *magic;
 #endif
@@ -131,7 +131,7 @@ do {								\
  */
 static inline bool rwsem_is_contended(struct rw_semaphore *sem)
 {
-	return sem->first_waiter != NULL;
+	return data_race(sem->first_waiter != NULL);
 }
 
 #if defined(CONFIG_DEBUG_RWSEMS) || defined(CONFIG_DETECT_HUNG_TASK_BLOCKER)
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -6,6 +6,7 @@ KCOV_INSTRUMENT		:= n
 CONTEXT_ANALYSIS_mutex.o := y
 CONTEXT_ANALYSIS_rtmutex_api.o := y
 CONTEXT_ANALYSIS_ww_rt_mutex.o := y
+CONTEXT_ANALYSIS_rwsem.o := y
 
 obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o
 
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -186,6 +186,7 @@ static __always_inline void rwbase_read_
 
 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
 					 unsigned long flags)
+	__releases(&rwb->rtmutex.wait_lock)
 {
 	struct rt_mutex_base *rtm = &rwb->rtmutex;
 
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -320,9 +320,10 @@ void __init_rwsem(struct rw_semaphore *s
 	sem->magic = sem;
 #endif
 	atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
-	raw_spin_lock_init(&sem->wait_lock);
-	sem->first_waiter = NULL;
 	atomic_long_set(&sem->owner, 0L);
+	scoped_guard (raw_spinlock_init, &sem->wait_lock) {
+		sem->first_waiter = NULL;
+	}
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 	osq_lock_init(&sem->osq);
 #endif
@@ -365,6 +366,7 @@ enum rwsem_wake_type {
 
 static inline
 bool __rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
+	__must_hold(&sem->wait_lock)
 {
 	if (list_empty(&waiter->list)) {
 		sem->first_waiter = NULL;
@@ -401,6 +403,7 @@ rwsem_del_waiter(struct rw_semaphore *se
 static inline
 struct rwsem_waiter *next_waiter(const struct rw_semaphore *sem,
 				 const struct rwsem_waiter *waiter)
+	__must_hold(&sem->wait_lock)
 {
 	struct rwsem_waiter *next = list_first_entry(&waiter->list,
 						     struct rwsem_waiter, list);
@@ -621,6 +624,7 @@ rwsem_del_wake_waiter(struct rw_semaphor
  */
 static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 					struct rwsem_waiter *waiter)
+	__must_hold(&sem->wait_lock)
 {
 	struct rwsem_waiter *first = sem->first_waiter;
 	long count, new;
@@ -1558,6 +1562,7 @@ static inline bool is_rwsem_reader_owned
  * lock for reading
  */
 void __sched down_read(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
@@ -1567,6 +1572,7 @@ void __sched down_read(struct rw_semapho
 EXPORT_SYMBOL(down_read);
 
 int __sched down_read_interruptible(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
@@ -1581,6 +1587,7 @@ int __sched down_read_interruptible(stru
 EXPORT_SYMBOL(down_read_interruptible);
 
 int __sched down_read_killable(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
@@ -1598,6 +1605,7 @@ EXPORT_SYMBOL(down_read_killable);
  * trylock for reading -- returns 1 if successful, 0 if contention
  */
 int down_read_trylock(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	int ret = __down_read_trylock(sem);
 
@@ -1611,6 +1619,7 @@ EXPORT_SYMBOL(down_read_trylock);
  * lock for writing
  */
 void __sched down_write(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
@@ -1622,6 +1631,7 @@ EXPORT_SYMBOL(down_write);
  * lock for writing
  */
 int __sched down_write_killable(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
@@ -1640,6 +1650,7 @@ EXPORT_SYMBOL(down_write_killable);
  * trylock for writing -- returns 1 if successful, 0 if contention
  */
 int down_write_trylock(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	int ret = __down_write_trylock(sem);
 
@@ -1654,6 +1665,7 @@ EXPORT_SYMBOL(down_write_trylock);
  * release a read lock
  */
 void up_read(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	rwsem_release(&sem->dep_map, _RET_IP_);
 	__up_read(sem);
@@ -1664,6 +1676,7 @@ EXPORT_SYMBOL(up_read);
  * release a write lock
  */
 void up_write(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	rwsem_release(&sem->dep_map, _RET_IP_);
 	__up_write(sem);
@@ -1674,6 +1687,7 @@ EXPORT_SYMBOL(up_write);
  * downgrade write lock to read lock
  */
 void downgrade_write(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	lock_downgrade(&sem->dep_map, _RET_IP_);
 	__downgrade_write(sem);
@@ -1683,6 +1697,7 @@ EXPORT_SYMBOL(downgrade_write);
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 void down_read_nested(struct rw_semaphore *sem, int subclass)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
@@ -1691,6 +1706,7 @@ void down_read_nested(struct rw_semaphor
 EXPORT_SYMBOL(down_read_nested);
 
 int down_read_killable_nested(struct rw_semaphore *sem, int subclass)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
@@ -1705,6 +1721,7 @@ int down_read_killable_nested(struct rw_
 EXPORT_SYMBOL(down_read_killable_nested);
 
 void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
@@ -1713,6 +1730,7 @@ void _down_write_nest_lock(struct rw_sem
 EXPORT_SYMBOL(_down_write_nest_lock);
 
 void down_read_non_owner(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	might_sleep();
 	__down_read(sem);
@@ -1727,6 +1745,7 @@ void down_read_non_owner(struct rw_semap
 EXPORT_SYMBOL(down_read_non_owner);
 
 void down_write_nested(struct rw_semaphore *sem, int subclass)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
@@ -1735,6 +1754,7 @@ void down_write_nested(struct rw_semapho
 EXPORT_SYMBOL(down_write_nested);
 
 int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
+	__no_context_analysis
 {
 	might_sleep();
 	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
@@ -1750,6 +1770,7 @@ int __sched down_write_killable_nested(s
 EXPORT_SYMBOL(down_write_killable_nested);
 
 void up_read_non_owner(struct rw_semaphore *sem)
+	__no_context_analysis
 {
 	DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 	__up_read(sem);