diff options
Diffstat (limited to 'lib/rwsem.c')
| -rw-r--r-- | lib/rwsem.c | 229 |
1 files changed, 120 insertions, 109 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c index 8337e1b9bb8d..19c5fa95e0b4 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -2,6 +2,9 @@ * * Written by David Howells (dhowells@redhat.com). * Derived from arch/i386/kernel/semaphore.c + * + * Writer lock-stealing by Alex Shi <alex.shi@intel.com> + * and Michel Lespinasse <walken@google.com> */ #include <linux/rwsem.h> #include <linux/sched.h> @@ -28,21 +31,22 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name, EXPORT_SYMBOL(__init_rwsem); +enum rwsem_waiter_type { + RWSEM_WAITING_FOR_WRITE, + RWSEM_WAITING_FOR_READ +}; + struct rwsem_waiter { struct list_head list; struct task_struct *task; - unsigned int flags; -#define RWSEM_WAITING_FOR_READ 0x00000001 -#define RWSEM_WAITING_FOR_WRITE 0x00000002 + enum rwsem_waiter_type type; }; -/* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and - * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held - * since the rwsem value was observed. - */ -#define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */ -#define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */ -#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */ +enum rwsem_wake_type { + RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */ + RWSEM_WAKE_READERS, /* Wake readers only */ + RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */ +}; /* * handle the lock release when processes blocked on it that can now run @@ -55,68 +59,43 @@ struct rwsem_waiter { * - writers are only woken if downgrading is false */ static struct rw_semaphore * -__rwsem_do_wake(struct rw_semaphore *sem, int wake_type) +__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) { struct rwsem_waiter *waiter; struct task_struct *tsk; struct list_head *next; - signed long oldcount, woken, loop, adjustment; + long oldcount, woken, loop, adjustment; waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); - if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) - goto readers_only; - - if (wake_type == RWSEM_WAKE_READ_OWNED) - /* Another active reader was observed, so wakeup is not - * likely to succeed. Save the atomic op. - */ + if (waiter->type == RWSEM_WAITING_FOR_WRITE) { + if (wake_type == RWSEM_WAKE_ANY) + /* Wake writer at the front of the queue, but do not + * grant it the lock yet as we want other writers + * to be able to steal it. Readers, on the other hand, + * will block as they will notice the queued writer. + */ + wake_up_process(waiter->task); goto out; + } - /* There's a writer at the front of the queue - try to grant it the - * write lock. However, we only wake this writer if we can transition - * the active part of the count from 0 -> 1 - */ - adjustment = RWSEM_ACTIVE_WRITE_BIAS; - if (waiter->list.next == &sem->wait_list) - adjustment -= RWSEM_WAITING_BIAS; - - try_again_write: - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; - if (oldcount & RWSEM_ACTIVE_MASK) - /* Someone grabbed the sem already */ - goto undo_write; - - /* We must be careful not to touch 'waiter' after we set ->task = NULL. - * It is an allocated on the waiter's stack and may become invalid at - * any time after that point (due to a wakeup from another source). + /* Writers might steal the lock before we grant it to the next reader. + * We prefer to do the first reader grant before counting readers + * so we can bail out early if a writer stole the lock. */ - list_del(&waiter->list); - tsk = waiter->task; - smp_mb(); - waiter->task = NULL; - wake_up_process(tsk); - put_task_struct(tsk); - goto out; - - readers_only: - /* If we come here from up_xxxx(), another thread might have reached - * rwsem_down_failed_common() before we acquired the spinlock and - * woken up a waiter, making it now active. We prefer to check for - * this first in order to not spend too much time with the spinlock - * held if we're not going to be able to wake up readers in the end. - * - * Note that we do not need to update the rwsem count: any writer - * trying to acquire rwsem will run rwsem_down_write_failed() due - * to the waiting threads and block trying to acquire the spinlock. - * - * We use a dummy atomic update in order to acquire the cache line - * exclusively since we expect to succeed and run the final rwsem - * count adjustment pretty soon. - */ - if (wake_type == RWSEM_WAKE_ANY && - rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS) - /* Someone grabbed the sem for write already */ - goto out; + adjustment = 0; + if (wake_type != RWSEM_WAKE_READ_OWNED) { + adjustment = RWSEM_ACTIVE_READ_BIAS; + try_reader_grant: + oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; + if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { + /* A writer stole the lock. Undo our reader grant. */ + if (rwsem_atomic_update(-adjustment, sem) & + RWSEM_ACTIVE_MASK) + goto out; + /* Last active locker left. Retry waking readers. */ + goto try_reader_grant; + } + } /* Grant an infinite number of read locks to the readers at the front * of the queue. Note we increment the 'active part' of the count by @@ -132,17 +111,19 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) waiter = list_entry(waiter->list.next, struct rwsem_waiter, list); - } while (waiter->flags & RWSEM_WAITING_FOR_READ); + } while (waiter->type != RWSEM_WAITING_FOR_WRITE); - adjustment = woken * RWSEM_ACTIVE_READ_BIAS; - if (waiter->flags & RWSEM_WAITING_FOR_READ) + adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; + if (waiter->type != RWSEM_WAITING_FOR_WRITE) /* hit end of list above */ adjustment -= RWSEM_WAITING_BIAS; - rwsem_atomic_add(adjustment, sem); + if (adjustment) + rwsem_atomic_add(adjustment, sem); next = sem->wait_list.next; - for (loop = woken; loop > 0; loop--) { + loop = woken; + do { waiter = list_entry(next, struct rwsem_waiter, list); next = waiter->list.next; tsk = waiter->task; @@ -150,41 +131,30 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) waiter->task = NULL; wake_up_process(tsk); put_task_struct(tsk); - } + } while (--loop); sem->wait_list.next = next; next->prev = &sem->wait_list; out: return sem; - - /* undo the change to the active count, but check for a transition - * 1->0 */ - undo_write: - if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) - goto out; - goto try_again_write; } /* - * wait for a lock to be granted + * wait for the read lock to be granted */ -static struct rw_semaphore __sched * -rwsem_down_failed_common(struct rw_semaphore *sem, - unsigned int flags, signed long adjustment) +struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) { + long count, adjustment = -RWSEM_ACTIVE_READ_BIAS; struct rwsem_waiter waiter; struct task_struct *tsk = current; - signed long count; - - set_task_state(tsk, TASK_UNINTERRUPTIBLE); /* set up my own style of waitqueue */ - raw_spin_lock_irq(&sem->wait_lock); waiter.task = tsk; - waiter.flags = flags; + waiter.type = RWSEM_WAITING_FOR_READ; get_task_struct(tsk); + raw_spin_lock_irq(&sem->wait_lock); if (list_empty(&sem->wait_list)) adjustment += RWSEM_WAITING_BIAS; list_add_tail(&waiter.list, &sem->wait_list); @@ -192,26 +162,24 @@ rwsem_down_failed_common(struct rw_semaphore *sem, /* we're now waiting on the lock, but no longer actively locking */ count = rwsem_atomic_update(adjustment, sem); - /* If there are no active locks, wake the front queued process(es) up. + /* If there are no active locks, wake the front queued process(es). * - * Alternatively, if we're called from a failed down_write(), there - * were already threads queued before us and there are no active - * writers, the lock must be read owned; so we try to wake any read - * locks that were queued ahead of us. */ - if (count == RWSEM_WAITING_BIAS) - sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); - else if (count > RWSEM_WAITING_BIAS && - adjustment == -RWSEM_ACTIVE_WRITE_BIAS) - sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); + * If there are no writers and we are first in the queue, + * wake our own waiter to join the existing active readers ! + */ + if (count == RWSEM_WAITING_BIAS || + (count > RWSEM_WAITING_BIAS && + adjustment != -RWSEM_ACTIVE_READ_BIAS)) + sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); raw_spin_unlock_irq(&sem->wait_lock); /* wait to be given the lock */ - for (;;) { + while (true) { + set_task_state(tsk, TASK_UNINTERRUPTIBLE); if (!waiter.task) break; schedule(); - set_task_state(tsk, TASK_UNINTERRUPTIBLE); } tsk->state = TASK_RUNNING; @@ -220,21 +188,64 @@ rwsem_down_failed_common(struct rw_semaphore *sem, } /* - * wait for the read lock to be granted - */ -struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) -{ - return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ, - -RWSEM_ACTIVE_READ_BIAS); -} - -/* - * wait for the write lock to be granted + * wait until we successfully acquire the write lock */ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) { - return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE, - -RWSEM_ACTIVE_WRITE_BIAS); + long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS; + struct rwsem_waiter waiter; + struct task_struct *tsk = current; + + /* set up my own style of waitqueue */ + waiter.task = tsk; + waiter.type = RWSEM_WAITING_FOR_WRITE; + + raw_spin_lock_irq(&sem->wait_lock); + if (list_empty(&sem->wait_list)) + adjustment += RWSEM_WAITING_BIAS; + list_add_tail(&waiter.list, &sem->wait_list); + + /* we're now waiting on the lock, but no longer actively locking */ + count = rwsem_atomic_update(adjustment, sem); + + /* If there were already threads queued before us and there are no + * active writers, the lock must be read owned; so we try to wake + * any read locks that were queued ahead of us. */ + if (count > RWSEM_WAITING_BIAS && + adjustment == -RWSEM_ACTIVE_WRITE_BIAS) + sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS); + + /* wait until we successfully acquire the lock */ + set_task_state(tsk, TASK_UNINTERRUPTIBLE); + while (true) { + if (!(count & RWSEM_ACTIVE_MASK)) { + /* Try acquiring the write lock. */ + count = RWSEM_ACTIVE_WRITE_BIAS; + if (!list_is_singular(&sem->wait_list)) + count += RWSEM_WAITING_BIAS; + + if (sem->count == RWSEM_WAITING_BIAS && + cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) == + RWSEM_WAITING_BIAS) + break; + } + + raw_spin_unlock_irq(&sem->wait_lock); + + /* Block until there are no active lockers. */ + do { + schedule(); + set_task_state(tsk, TASK_UNINTERRUPTIBLE); + } while ((count = sem->count) & RWSEM_ACTIVE_MASK); + + raw_spin_lock_irq(&sem->wait_lock); + } + + list_del(&waiter.list); + raw_spin_unlock_irq(&sem->wait_lock); + tsk->state = TASK_RUNNING; + + return sem; } /* |