/* rwsem-spinlock.c: R/W semaphores: contention handling functions for * generic spinlock implementation * * Copyright (c) 2001 David Howells (dhowells@redhat.com). * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de> * - Derived also from comments by Linus */ #include <linux/rwsem.h> #include <linux/sched/signal.h> #include <linux/sched/debug.h> #include <linux/export.h> enum rwsem_waiter_type { RWSEM_WAITING_FOR_WRITE, RWSEM_WAITING_FOR_READ }; struct rwsem_waiter { struct list_head list; struct task_struct *task; enum rwsem_waiter_type type; }; int rwsem_is_locked(struct rw_semaphore *sem) { int ret = 1; unsigned long flags; if (raw_spin_trylock_irqsave(&sem->wait_lock, flags)) { ret = (sem->count != 0); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); } return ret; } EXPORT_SYMBOL(rwsem_is_locked); /* * initialise the semaphore */ void __init_rwsem(struct rw_semaphore *sem, const char *name, struct lock_class_key *key) { #ifdef CONFIG_DEBUG_LOCK_ALLOC /* * Make sure we are not reinitializing a held semaphore: */ debug_check_no_locks_freed((void *)sem, sizeof(*sem)); lockdep_init_map(&sem->dep_map, name, key, 0); #endif sem->count = 0; raw_spin_lock_init(&sem->wait_lock); INIT_LIST_HEAD(&sem->wait_list); } EXPORT_SYMBOL(__init_rwsem); /* * handle the lock release when processes blocked on it that can now run * - if we come here, then: * - the 'active count' _reached_ zero * - the 'waiting count' is non-zero * - the spinlock must be held by the caller * - woken process blocks are discarded from the list after having task zeroed * - writers are only woken if wakewrite is non-zero */ static inline struct rw_semaphore * __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite) { struct rwsem_waiter *waiter; struct task_struct *tsk; int woken; waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); if (waiter->type == RWSEM_WAITING_FOR_WRITE) { if (wakewrite) /* Wake up a writer. Note that we do not grant it the * lock - it will have to acquire it when it runs. */ wake_up_process(waiter->task); goto out; } /* grant an infinite number of read locks to the front of the queue */ woken = 0; do { struct list_head *next = waiter->list.next; list_del(&waiter->list); tsk = waiter->task; /* * Make sure we do not wakeup the next reader before * setting the nil condition to grant the next reader; * otherwise we could miss the wakeup on the other * side and end up sleeping again. See the pairing * in rwsem_down_read_failed(). */ smp_mb(); waiter->task = NULL; wake_up_process(tsk); put_task_struct(tsk); woken++; if (next == &sem->wait_list) break; waiter = list_entry(next, struct rwsem_waiter, list); } while (waiter->type != RWSEM_WAITING_FOR_WRITE); sem->count += woken; out: return sem; } /* * wake a single writer */ static inline struct rw_semaphore * __rwsem_wake_one_writer(struct rw_semaphore *sem) { struct rwsem_waiter *waiter; waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); wake_up_process(waiter->task); return sem; } /* * get a read lock on the semaphore */ void __sched __down_read(struct rw_semaphore *sem) { struct rwsem_waiter waiter; unsigned long flags; raw_spin_lock_irqsave(&sem->wait_lock, flags); if (sem->count >= 0 && list_empty(&sem->wait_list)) { /* granted */ sem->count++; raw_spin_unlock_irqrestore(&sem->wait_lock, flags); goto out; } set_current_state(TASK_UNINTERRUPTIBLE); /* set up my own style of waitqueue */ waiter.task = current; waiter.type = RWSEM_WAITING_FOR_READ; get_task_struct(current); list_add_tail(&waiter.list, &sem->wait_list); /* we don't need to touch the semaphore struct anymore */ raw_spin_unlock_irqrestore(&sem->wait_lock, flags); /* wait to be given the lock */ for (;;) { if (!waiter.task) break; schedule(); set_current_state(TASK_UNINTERRUPTIBLE); } __set_current_state(TASK_RUNNING); out: ; } /* * trylock for reading -- returns 1 if successful, 0 if contention */ int __down_read_trylock(struct rw_semaphore *sem) { unsigned long flags; int ret = 0; raw_spin_lock_irqsave(&sem->wait_lock, flags); if (sem->count >= 0 && list_empty(&sem->wait_list)) { /* granted */ sem->count++; ret = 1; } raw_spin_unlock_irqrestore(&sem->wait_lock, flags); return ret; } /* * get a write lock on the semaphore */ int __sched __down_write_common(struct rw_semaphore *sem, int state) { struct rwsem_waiter waiter; unsigned long flags; int ret = 0; raw_spin_lock_irqsave(&sem->wait_lock, flags); /* set up my own style of waitqueue */ waiter.task = current; waiter.type = RWSEM_WAITING_FOR_WRITE; list_add_tail(&waiter.list, &sem->wait_list); /* wait for someone to release the lock */ for (;;) { /* * That is the key to support write lock stealing: allows the * task already on CPU to get the lock soon rather than put * itself into sleep and waiting for system woke it or someone * else in the head of the wait list up. */ if (sem->count == 0) break; if (signal_pending_state(state, current)) goto out_nolock; set_current_state(state); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); schedule(); raw_spin_lock_irqsave(&sem->wait_lock, flags); } /* got the lock */ sem->count = -1; list_del(&waiter.list); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); return ret; out_nolock: list_del(&waiter.list); if (!list_empty(&sem->wait_list) && sem->count >= 0) __rwsem_do_wake(sem, 0); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); return -EINTR; } void __sched __down_write(struct rw_semaphore *sem) { __down_write_common(sem, TASK_UNINTERRUPTIBLE); } int __sched __down_write_killable(struct rw_semaphore *sem) { return __down_write_common(sem, TASK_KILLABLE); } /* * trylock for writing -- returns 1 if successful, 0 if contention */ int __down_write_trylock(struct rw_semaphore *sem) { unsigned long flags; int ret = 0; raw_spin_lock_irqsave(&sem->wait_lock, flags); if (sem->count == 0) { /* got the lock */ sem->count = -1; ret = 1; } raw_spin_unlock_irqrestore(&sem->wait_lock, flags); return ret; } /* * release a read lock on the semaphore */ void __up_read(struct rw_semaphore *sem) { unsigned long flags; raw_spin_lock_irqsave(&sem->wait_lock, flags); if (--sem->count == 0 && !list_empty(&sem->wait_list)) sem = __rwsem_wake_one_writer(sem); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); } /* * release a write lock on the semaphore */ void __up_write(struct rw_semaphore *sem) { unsigned long flags; raw_spin_lock_irqsave(&sem->wait_lock, flags); sem->count = 0; if (!list_empty(&sem->wait_list)) sem = __rwsem_do_wake(sem, 1); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); } /* * downgrade a write lock into a read lock * - just wake up any readers at the front of the queue */ void __downgrade_write(struct rw_semaphore *sem) { unsigned long flags; raw_spin_lock_irqsave(&sem->wait_lock, flags); sem->count = 1; if (!list_empty(&sem->wait_list)) sem = __rwsem_do_wake(sem, 0); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); }