diff options
Diffstat (limited to 'kernel/locking/percpu-rwsem.c')
| -rw-r--r-- | kernel/locking/percpu-rwsem.c | 194 |
1 files changed, 134 insertions, 60 deletions
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c index 364d38a0c444..a008a1ba21a7 100644 --- a/kernel/locking/percpu-rwsem.c +++ b/kernel/locking/percpu-rwsem.c @@ -1,27 +1,29 @@ // SPDX-License-Identifier: GPL-2.0-only #include <linux/atomic.h> -#include <linux/rwsem.h> #include <linux/percpu.h> +#include <linux/wait.h> #include <linux/lockdep.h> #include <linux/percpu-rwsem.h> #include <linux/rcupdate.h> #include <linux/sched.h> +#include <linux/sched/task.h> #include <linux/errno.h> -#include "rwsem.h" - int __percpu_init_rwsem(struct percpu_rw_semaphore *sem, - const char *name, struct lock_class_key *rwsem_key) + const char *name, struct lock_class_key *key) { sem->read_count = alloc_percpu(int); if (unlikely(!sem->read_count)) return -ENOMEM; - /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */ rcu_sync_init(&sem->rss); - __init_rwsem(&sem->rw_sem, name, rwsem_key); rcuwait_init(&sem->writer); - sem->readers_block = 0; + init_waitqueue_head(&sem->waiters); + atomic_set(&sem->block, 0); +#ifdef CONFIG_DEBUG_LOCK_ALLOC + debug_check_no_locks_freed((void *)sem, sizeof(*sem)); + lockdep_init_map(&sem->dep_map, name, key, 0); +#endif return 0; } EXPORT_SYMBOL_GPL(__percpu_init_rwsem); @@ -41,73 +43,139 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *sem) } EXPORT_SYMBOL_GPL(percpu_free_rwsem); -int __percpu_down_read(struct percpu_rw_semaphore *sem, int try) +static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem) { + __this_cpu_inc(*sem->read_count); + /* * Due to having preemption disabled the decrement happens on * the same CPU as the increment, avoiding the * increment-on-one-CPU-and-decrement-on-another problem. * - * If the reader misses the writer's assignment of readers_block, then - * the writer is guaranteed to see the reader's increment. + * If the reader misses the writer's assignment of sem->block, then the + * writer is guaranteed to see the reader's increment. * * Conversely, any readers that increment their sem->read_count after - * the writer looks are guaranteed to see the readers_block value, - * which in turn means that they are guaranteed to immediately - * decrement their sem->read_count, so that it doesn't matter that the - * writer missed them. + * the writer looks are guaranteed to see the sem->block value, which + * in turn means that they are guaranteed to immediately decrement + * their sem->read_count, so that it doesn't matter that the writer + * missed them. */ smp_mb(); /* A matches D */ /* - * If !readers_block the critical section starts here, matched by the + * If !sem->block the critical section starts here, matched by the * release in percpu_up_write(). */ - if (likely(!smp_load_acquire(&sem->readers_block))) + if (likely(!atomic_read_acquire(&sem->block))) + return true; + + __this_cpu_dec(*sem->read_count); + + /* Prod writer to re-evaluate readers_active_check() */ + rcuwait_wake_up(&sem->writer); + + return false; +} + +static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem) +{ + if (atomic_read(&sem->block)) + return false; + + return atomic_xchg(&sem->block, 1) == 0; +} + +static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader) +{ + if (reader) { + bool ret; + + preempt_disable(); + ret = __percpu_down_read_trylock(sem); + preempt_enable(); + + return ret; + } + return __percpu_down_write_trylock(sem); +} + +/* + * The return value of wait_queue_entry::func means: + * + * <0 - error, wakeup is terminated and the error is returned + * 0 - no wakeup, a next waiter is tried + * >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive. + * + * We use EXCLUSIVE for both readers and writers to preserve FIFO order, + * and play games with the return value to allow waking multiple readers. + * + * Specifically, we wake readers until we've woken a single writer, or until a + * trylock fails. + */ +static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry, + unsigned int mode, int wake_flags, + void *key) +{ + struct task_struct *p = get_task_struct(wq_entry->private); + bool reader = wq_entry->flags & WQ_FLAG_CUSTOM; + struct percpu_rw_semaphore *sem = key; + + /* concurrent against percpu_down_write(), can get stolen */ + if (!__percpu_rwsem_trylock(sem, reader)) return 1; - /* - * Per the above comment; we still have preemption disabled and - * will thus decrement on the same CPU as we incremented. - */ - __percpu_up_read(sem); + list_del_init(&wq_entry->entry); + smp_store_release(&wq_entry->private, NULL); - if (try) - return 0; + wake_up_process(p); + put_task_struct(p); - /* - * We either call schedule() in the wait, or we'll fall through - * and reschedule on the preempt_enable() in percpu_down_read(). - */ - preempt_enable_no_resched(); + return !reader; /* wake (readers until) 1 writer */ +} + +static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader) +{ + DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function); + bool wait; + spin_lock_irq(&sem->waiters.lock); /* - * Avoid lockdep for the down/up_read() we already have them. + * Serialize against the wakeup in percpu_up_write(), if we fail + * the trylock, the wakeup must see us on the list. */ - __down_read(&sem->rw_sem); - this_cpu_inc(*sem->read_count); - __up_read(&sem->rw_sem); + wait = !__percpu_rwsem_trylock(sem, reader); + if (wait) { + wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM; + __add_wait_queue_entry_tail(&sem->waiters, &wq_entry); + } + spin_unlock_irq(&sem->waiters.lock); - preempt_disable(); - return 1; + while (wait) { + set_current_state(TASK_UNINTERRUPTIBLE); + if (!smp_load_acquire(&wq_entry.private)) + break; + schedule(); + } + __set_current_state(TASK_RUNNING); } -EXPORT_SYMBOL_GPL(__percpu_down_read); -void __percpu_up_read(struct percpu_rw_semaphore *sem) +bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try) { - smp_mb(); /* B matches C */ - /* - * In other words, if they see our decrement (presumably to aggregate - * zero, as that is the only time it matters) they will also see our - * critical section. - */ - __this_cpu_dec(*sem->read_count); + if (__percpu_down_read_trylock(sem)) + return true; - /* Prod writer to recheck readers_active */ - rcuwait_wake_up(&sem->writer); + if (try) + return false; + + preempt_enable(); + percpu_rwsem_wait(sem, /* .reader = */ true); + preempt_disable(); + + return true; } -EXPORT_SYMBOL_GPL(__percpu_up_read); +EXPORT_SYMBOL_GPL(__percpu_down_read); #define per_cpu_sum(var) \ ({ \ @@ -124,6 +192,8 @@ EXPORT_SYMBOL_GPL(__percpu_up_read); * zero. If this sum is zero, then it is stable due to the fact that if any * newly arriving readers increment a given counter, they will immediately * decrement that same counter. + * + * Assumes sem->block is set. */ static bool readers_active_check(struct percpu_rw_semaphore *sem) { @@ -142,32 +212,36 @@ static bool readers_active_check(struct percpu_rw_semaphore *sem) void percpu_down_write(struct percpu_rw_semaphore *sem) { + might_sleep(); + rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_); + /* Notify readers to take the slow path. */ rcu_sync_enter(&sem->rss); - down_write(&sem->rw_sem); - /* - * Notify new readers to block; up until now, and thus throughout the - * longish rcu_sync_enter() above, new readers could still come in. + * Try set sem->block; this provides writer-writer exclusion. + * Having sem->block set makes new readers block. */ - WRITE_ONCE(sem->readers_block, 1); + if (!__percpu_down_write_trylock(sem)) + percpu_rwsem_wait(sem, /* .reader = */ false); - smp_mb(); /* D matches A */ + /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */ /* - * If they don't see our writer of readers_block, then we are - * guaranteed to see their sem->read_count increment, and therefore - * will wait for them. + * If they don't see our store of sem->block, then we are guaranteed to + * see their sem->read_count increment, and therefore will wait for + * them. */ - /* Wait for all now active readers to complete. */ - rcuwait_wait_event(&sem->writer, readers_active_check(sem)); + /* Wait for all active readers to complete. */ + rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE); } EXPORT_SYMBOL_GPL(percpu_down_write); void percpu_up_write(struct percpu_rw_semaphore *sem) { + rwsem_release(&sem->dep_map, _RET_IP_); + /* * Signal the writer is done, no fast path yet. * @@ -178,12 +252,12 @@ void percpu_up_write(struct percpu_rw_semaphore *sem) * Therefore we force it through the slow path which guarantees an * acquire and thereby guarantees the critical section's consistency. */ - smp_store_release(&sem->readers_block, 0); + atomic_set_release(&sem->block, 0); /* - * Release the write lock, this will allow readers back in the game. + * Prod any pending reader/writer to make progress. */ - up_write(&sem->rw_sem); + __wake_up(&sem->waiters, TASK_NORMAL, 1, sem); /* * Once this completes (at least one RCU-sched grace period hence) the |