diff options
Diffstat (limited to 'kernel/locking/rwsem.h')
| -rw-r--r-- | kernel/locking/rwsem.h | 95 |
1 files changed, 71 insertions, 24 deletions
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h index b9d0e72aa80f..bad2bca0268b 100644 --- a/kernel/locking/rwsem.h +++ b/kernel/locking/rwsem.h @@ -1,24 +1,30 @@ /* SPDX-License-Identifier: GPL-2.0 */ /* - * The owner field of the rw_semaphore structure will be set to - * RWSEM_READER_OWNED when a reader grabs the lock. A writer will clear - * the owner field when it unlocks. A reader, on the other hand, will - * not touch the owner field when it unlocks. + * The least significant 2 bits of the owner value has the following + * meanings when set. + * - RWSEM_READER_OWNED (bit 0): The rwsem is owned by readers + * - RWSEM_ANONYMOUSLY_OWNED (bit 1): The rwsem is anonymously owned, + * i.e. the owner(s) cannot be readily determined. It can be reader + * owned or the owning writer is indeterminate. * - * In essence, the owner field now has the following 4 states: - * 1) 0 - * - lock is free or the owner hasn't set the field yet - * 2) RWSEM_READER_OWNED - * - lock is currently or previously owned by readers (lock is free - * or not set by owner yet) - * 3) RWSEM_ANONYMOUSLY_OWNED bit set with some other bits set as well - * - lock is owned by an anonymous writer, so spinning on the lock - * owner should be disabled. - * 4) Other non-zero value - * - a writer owns the lock and other writers can spin on the lock owner. + * When a writer acquires a rwsem, it puts its task_struct pointer + * into the owner field. It is cleared after an unlock. + * + * When a reader acquires a rwsem, it will also puts its task_struct + * pointer into the owner field with both the RWSEM_READER_OWNED and + * RWSEM_ANONYMOUSLY_OWNED bits set. On unlock, the owner field will + * largely be left untouched. So for a free or reader-owned rwsem, + * the owner value may contain information about the last reader that + * acquires the rwsem. The anonymous bit is set because that particular + * reader may or may not still own the lock. + * + * That information may be helpful in debugging cases where the system + * seems to hang on a reader owned rwsem especially if only one reader + * is involved. Ideally we would like to track all the readers that own + * a rwsem, but the overhead is simply too big. */ -#define RWSEM_ANONYMOUSLY_OWNED (1UL << 0) -#define RWSEM_READER_OWNED ((struct task_struct *)RWSEM_ANONYMOUSLY_OWNED) +#define RWSEM_READER_OWNED (1UL << 0) +#define RWSEM_ANONYMOUSLY_OWNED (1UL << 1) #ifdef CONFIG_DEBUG_RWSEMS # define DEBUG_RWSEMS_WARN_ON(c) DEBUG_LOCKS_WARN_ON(c) @@ -44,15 +50,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem) WRITE_ONCE(sem->owner, NULL); } +/* + * The task_struct pointer of the last owning reader will be left in + * the owner field. + * + * Note that the owner value just indicates the task has owned the rwsem + * previously, it may not be the real owner or one of the real owners + * anymore when that field is examined, so take it with a grain of salt. + */ +static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem, + struct task_struct *owner) +{ + unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED + | RWSEM_ANONYMOUSLY_OWNED; + + WRITE_ONCE(sem->owner, (struct task_struct *)val); +} + static inline void rwsem_set_reader_owned(struct rw_semaphore *sem) { - /* - * We check the owner value first to make sure that we will only - * do a write to the rwsem cacheline when it is really necessary - * to minimize cacheline contention. - */ - if (READ_ONCE(sem->owner) != RWSEM_READER_OWNED) - WRITE_ONCE(sem->owner, RWSEM_READER_OWNED); + __rwsem_set_reader_owned(sem, current); } /* @@ -72,6 +89,25 @@ static inline bool rwsem_has_anonymous_owner(struct task_struct *owner) { return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED; } + +#ifdef CONFIG_DEBUG_RWSEMS +/* + * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there + * is a task pointer in owner of a reader-owned rwsem, it will be the + * real owner or one of the real owners. The only exception is when the + * unlock is done by up_read_non_owner(). + */ +#define rwsem_clear_reader_owned rwsem_clear_reader_owned +static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem) +{ + unsigned long val = (unsigned long)current | RWSEM_READER_OWNED + | RWSEM_ANONYMOUSLY_OWNED; + if (READ_ONCE(sem->owner) == (struct task_struct *)val) + cmpxchg_relaxed((unsigned long *)&sem->owner, val, + RWSEM_READER_OWNED | RWSEM_ANONYMOUSLY_OWNED); +} +#endif + #else static inline void rwsem_set_owner(struct rw_semaphore *sem) { @@ -81,7 +117,18 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem) { } +static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem, + struct task_struct *owner) +{ +} + static inline void rwsem_set_reader_owned(struct rw_semaphore *sem) { } #endif + +#ifndef rwsem_clear_reader_owned +static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem) +{ +} +#endif |