diff options
Diffstat (limited to 'kernel/locking/rwsem-xadd.c')
| -rw-r--r-- | kernel/locking/rwsem-xadd.c | 56 |
1 files changed, 24 insertions, 32 deletions
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 75c950ede9c7..1be33caf157d 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -511,38 +511,30 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) unsigned long flags; /* - * If a spinner is present, there is a chance that the load of - * rwsem_has_spinner() in rwsem_wake() can be reordered with - * respect to decrement of rwsem count in __up_write() leading - * to wakeup being missed. - * - * spinning writer up_write caller - * --------------- ----------------------- - * [S] osq_unlock() [L] osq - * spin_lock(wait_lock) - * sem->count=0xFFFFFFFF00000001 - * +0xFFFFFFFF00000000 - * count=sem->count - * MB - * sem->count=0xFFFFFFFE00000001 - * -0xFFFFFFFF00000001 - * RMB - * spin_trylock(wait_lock) - * return - * rwsem_try_write_lock(count) - * spin_unlock(wait_lock) - * schedule() - * - * Reordering of atomic_long_sub_return_release() in __up_write() - * and rwsem_has_spinner() in rwsem_wake() can cause missing of - * wakeup in up_write() context. In spinning writer, sem->count - * and local variable count is 0XFFFFFFFE00000001. It would result - * in rwsem_try_write_lock() failing to acquire rwsem and spinning - * writer going to sleep in rwsem_down_write_failed(). - * - * The smp_rmb() here is to make sure that the spinner state is - * consulted after sem->count is updated in up_write context. - */ + * __rwsem_down_write_failed_common(sem) + * rwsem_optimistic_spin(sem) + * osq_unlock(sem->osq) + * ... + * atomic_long_add_return(&sem->count) + * + * - VS - + * + * __up_write() + * if (atomic_long_sub_return_release(&sem->count) < 0) + * rwsem_wake(sem) + * osq_is_locked(&sem->osq) + * + * And __up_write() must observe !osq_is_locked() when it observes the + * atomic_long_add_return() in order to not miss a wakeup. + * + * This boils down to: + * + * [S.rel] X = 1 [RmW] r0 = (Y += 0) + * MB RMB + * [RmW] Y += 1 [L] r1 = X + * + * exists (r0=1 /\ r1=0) + */ smp_rmb(); /* |