From 1ea4b473504b6dc6a0d21c298519aff2d52433c9 Mon Sep 17 00:00:00 2001 From: "Matthew Wilcox (Oracle)" Date: Thu, 5 Mar 2026 19:55:41 +0000 Subject: locking/rwsem: Remove the list_head from struct rw_semaphore Instead of embedding a list_head in struct rw_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, 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 some core data structures like struct inode. Signed-off-by: Matthew Wilcox (Oracle) Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260305195545.3707590-2-willy@infradead.org --- include/linux/rwsem.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) (limited to 'include/linux/rwsem.h') diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h index 9bf1d93d3d7b..e7829531c4ba 100644 --- 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 list_head wait_list; + struct rwsem_waiter *first_waiter; #ifdef CONFIG_DEBUG_RWSEMS void *magic; #endif @@ -106,7 +106,7 @@ static inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore * .owner = ATOMIC_LONG_INIT(0), \ __RWSEM_OPT_INIT(name) \ .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\ - .wait_list = LIST_HEAD_INIT((name).wait_list), \ + .first_waiter = NULL, \ __RWSEM_DEBUG_INIT(name) \ __RWSEM_DEP_MAP_INIT(name) } @@ -129,9 +129,9 @@ do { \ * rwsem to see if somebody from an incompatible type is wanting access to the * lock. */ -static inline int rwsem_is_contended(struct rw_semaphore *sem) +static inline bool rwsem_is_contended(struct rw_semaphore *sem) { - return !list_empty(&sem->wait_list); + return sem->first_waiter != NULL; } #if defined(CONFIG_DEBUG_RWSEMS) || defined(CONFIG_DETECT_HUNG_TASK_BLOCKER) -- cgit v1.2.3 From 739690915ce1f017223ef4e6f3cc966ccfa3c861 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 6 Mar 2026 10:43:56 +0100 Subject: locking/rwsem: Add context analysis Add compiler context analysis annotations. Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260306101417.GT1282955@noisy.programming.kicks-ass.net --- include/linux/rwsem.h | 4 ++-- kernel/locking/Makefile | 1 + kernel/locking/rwbase_rt.c | 1 + kernel/locking/rwsem.c | 27 ++++++++++++++++++++++++--- 4 files changed, 28 insertions(+), 5 deletions(-) (limited to 'include/linux/rwsem.h') diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h index e7829531c4ba..6a1a7bae5f81 100644 --- 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) diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile index 0c07de79388c..cee1901d4cff 100644 --- 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 diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c index 9f4322c07486..82e078c0665a 100644 --- a/kernel/locking/rwbase_rt.c +++ b/kernel/locking/rwbase_rt.c @@ -186,6 +186,7 @@ static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb, 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; diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index e66f37ebc6f6..ba4cb74de064 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -72,7 +72,7 @@ #c, atomic_long_read(&(sem)->count), \ (unsigned long) sem->magic, \ atomic_long_read(&(sem)->owner), (long)current, \ - (sem)->first_waiter ? "" : "not ")) \ + rwsem_is_contended(sem) ? "" : "not ")) \ debug_locks_off(); \ } while (0) #else @@ -320,9 +320,10 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name, 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 *sem, struct rwsem_waiter *waiter) 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_semaphore *sem, struct rwsem_waiter *waiter, */ 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(struct rw_semaphore *sem) * 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_semaphore *sem) 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(struct rw_semaphore *sem) 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_semaphore *sem, int subclass) 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_semaphore *sem, int subclass) 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_semaphore *sem, struct lockdep_map *nest) 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_semaphore *sem) 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_semaphore *sem, int subclass) 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(struct rw_semaphore *sem, int subclass) 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); -- cgit v1.2.3