202 lines
6.1 KiB
C
202 lines
6.1 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/* rwsem.h: R/W semaphores, public interface
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*
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* Written by David Howells (dhowells@redhat.com).
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* Derived from asm-i386/semaphore.h
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*/
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#ifndef _LINUX_RWSEM_H
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#define _LINUX_RWSEM_H
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#include <linux/linkage.h>
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#include <linux/types.h>
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#include <linux/kernel.h>
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#include <linux/list.h>
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#include <linux/spinlock.h>
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#include <linux/atomic.h>
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#include <linux/err.h>
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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#include <linux/osq_lock.h>
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#endif
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/*
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* For an uncontended rwsem, count and owner are the only fields a task
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* needs to touch when acquiring the rwsem. So they are put next to each
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* other to increase the chance that they will share the same cacheline.
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*
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* In a contended rwsem, the owner is likely the most frequently accessed
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* field in the structure as the optimistic waiter that holds the osq lock
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* will spin on owner. For an embedded rwsem, other hot fields in the
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* containing structure should be moved further away from the rwsem to
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* reduce the chance that they will share the same cacheline causing
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* cacheline bouncing problem.
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*/
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struct rw_semaphore {
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atomic_long_t count;
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/*
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* Write owner or one of the read owners as well flags regarding
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* the current state of the rwsem. Can be used as a speculative
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* check to see if the write owner is running on the cpu.
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*/
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atomic_long_t owner;
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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struct optimistic_spin_queue osq; /* spinner MCS lock */
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#endif
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raw_spinlock_t wait_lock;
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struct list_head wait_list;
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#ifdef CONFIG_DEBUG_RWSEMS
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void *magic;
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#endif
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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struct lockdep_map dep_map;
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#endif
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};
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/* In all implementations count != 0 means locked */
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static inline int rwsem_is_locked(struct rw_semaphore *sem)
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{
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return atomic_long_read(&sem->count) != 0;
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}
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#define RWSEM_UNLOCKED_VALUE 0L
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#define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
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/* Common initializer macros and functions */
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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# define __RWSEM_DEP_MAP_INIT(lockname) \
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, .dep_map = { \
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.name = #lockname, \
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.wait_type_inner = LD_WAIT_SLEEP, \
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}
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#else
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# define __RWSEM_DEP_MAP_INIT(lockname)
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#endif
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#ifdef CONFIG_DEBUG_RWSEMS
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# define __DEBUG_RWSEM_INITIALIZER(lockname) , .magic = &lockname
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#else
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# define __DEBUG_RWSEM_INITIALIZER(lockname)
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#endif
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED
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#else
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#define __RWSEM_OPT_INIT(lockname)
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#endif
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#define __RWSEM_INITIALIZER(name) \
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{ __RWSEM_INIT_COUNT(name), \
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.owner = ATOMIC_LONG_INIT(0), \
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.wait_list = LIST_HEAD_INIT((name).wait_list), \
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.wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
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__RWSEM_OPT_INIT(name) \
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__DEBUG_RWSEM_INITIALIZER(name) \
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__RWSEM_DEP_MAP_INIT(name) }
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#define DECLARE_RWSEM(name) \
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struct rw_semaphore name = __RWSEM_INITIALIZER(name)
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extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key);
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#define init_rwsem(sem) \
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do { \
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static struct lock_class_key __key; \
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\
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__init_rwsem((sem), #sem, &__key); \
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} while (0)
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/*
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* This is the same regardless of which rwsem implementation that is being used.
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* It is just a heuristic meant to be called by somebody alreadying holding the
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* rwsem to see if somebody from an incompatible type is wanting access to the
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* lock.
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*/
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static inline int rwsem_is_contended(struct rw_semaphore *sem)
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{
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return !list_empty(&sem->wait_list);
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}
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/*
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* lock for reading
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*/
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extern void down_read(struct rw_semaphore *sem);
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extern int __must_check down_read_killable(struct rw_semaphore *sem);
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/*
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* trylock for reading -- returns 1 if successful, 0 if contention
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*/
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extern int down_read_trylock(struct rw_semaphore *sem);
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/*
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* lock for writing
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*/
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extern void down_write(struct rw_semaphore *sem);
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extern int __must_check down_write_killable(struct rw_semaphore *sem);
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/*
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* trylock for writing -- returns 1 if successful, 0 if contention
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*/
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extern int down_write_trylock(struct rw_semaphore *sem);
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/*
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* release a read lock
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*/
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extern void up_read(struct rw_semaphore *sem);
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/*
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* release a write lock
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*/
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extern void up_write(struct rw_semaphore *sem);
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/*
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* downgrade write lock to read lock
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*/
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extern void downgrade_write(struct rw_semaphore *sem);
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* nested locking. NOTE: rwsems are not allowed to recurse
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* (which occurs if the same task tries to acquire the same
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* lock instance multiple times), but multiple locks of the
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* same lock class might be taken, if the order of the locks
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* is always the same. This ordering rule can be expressed
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* to lockdep via the _nested() APIs, but enumerating the
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* subclasses that are used. (If the nesting relationship is
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* static then another method for expressing nested locking is
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* the explicit definition of lock class keys and the use of
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* lockdep_set_class() at lock initialization time.
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* See Documentation/locking/lockdep-design.rst for more details.)
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*/
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extern void down_read_nested(struct rw_semaphore *sem, int subclass);
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extern void down_write_nested(struct rw_semaphore *sem, int subclass);
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extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
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extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
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# define down_write_nest_lock(sem, nest_lock) \
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do { \
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typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
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_down_write_nest_lock(sem, &(nest_lock)->dep_map); \
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} while (0);
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/*
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* Take/release a lock when not the owner will release it.
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*
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* [ This API should be avoided as much as possible - the
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* proper abstraction for this case is completions. ]
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*/
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extern void down_read_non_owner(struct rw_semaphore *sem);
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extern void up_read_non_owner(struct rw_semaphore *sem);
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#else
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# define down_read_nested(sem, subclass) down_read(sem)
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# define down_write_nest_lock(sem, nest_lock) down_write(sem)
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# define down_write_nested(sem, subclass) down_write(sem)
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# define down_write_killable_nested(sem, subclass) down_write_killable(sem)
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# define down_read_non_owner(sem) down_read(sem)
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# define up_read_non_owner(sem) up_read(sem)
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#endif
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#endif /* _LINUX_RWSEM_H */
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