diff options
| -rw-r--r-- | kernel/locking/rwsem-xadd.c | 46 |
1 files changed, 31 insertions, 15 deletions
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 6b3ee9948bf1..0b1f77957240 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -130,6 +130,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, { struct rwsem_waiter *waiter, *tmp; long oldcount, woken = 0, adjustment = 0; + struct list_head wlist; /* * Take a peek at the queue head waiter such that we can determine @@ -188,18 +189,43 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, * of the queue. We know that woken will be at least 1 as we accounted * for above. Note we increment the 'active part' of the count by the * number of readers before waking any processes up. + * + * We have to do wakeup in 2 passes to prevent the possibility that + * the reader count may be decremented before it is incremented. It + * is because the to-be-woken waiter may not have slept yet. So it + * may see waiter->task got cleared, finish its critical section and + * do an unlock before the reader count increment. + * + * 1) Collect the read-waiters in a separate list, count them and + * fully increment the reader count in rwsem. + * 2) For each waiters in the new list, clear waiter->task and + * put them into wake_q to be woken up later. */ - list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) { - struct task_struct *tsk; - + list_for_each_entry(waiter, &sem->wait_list, list) { if (waiter->type == RWSEM_WAITING_FOR_WRITE) break; woken++; - tsk = waiter->task; + } + list_cut_before(&wlist, &sem->wait_list, &waiter->list); + + adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; + lockevent_cond_inc(rwsem_wake_reader, woken); + if (list_empty(&sem->wait_list)) { + /* hit end of list above */ + adjustment -= RWSEM_WAITING_BIAS; + } + + if (adjustment) + atomic_long_add(adjustment, &sem->count); + + /* 2nd pass */ + list_for_each_entry_safe(waiter, tmp, &wlist, list) { + struct task_struct *tsk; + tsk = waiter->task; get_task_struct(tsk); - list_del(&waiter->list); + /* * Ensure calling get_task_struct() before setting the reader * waiter to nil such that rwsem_down_read_failed() cannot @@ -213,16 +239,6 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, */ wake_q_add_safe(wake_q, tsk); } - - adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; - lockevent_cond_inc(rwsem_wake_reader, woken); - if (list_empty(&sem->wait_list)) { - /* hit end of list above */ - adjustment -= RWSEM_WAITING_BIAS; - } - - if (adjustment) - atomic_long_add(adjustment, &sem->count); } /* |