Revert "Taskq locking optimizations"

This reverts commit ec2b41049f7f576aaa772b326d083e5971212d33.

A race condition was introduced by which a wake_up() call can be lost
after the taskq thread determines there is no pending work items,
leading to deadlock:

1. taksq thread enables interrupts
2. dispatcher thread runs, queues work item, call wake_up()
3. taskq thread runs, adds self to waitq, sleeps

This could easily happen if an interrupt for an IO completion was
outstanding at the point where the taskq thread reenables interrupts,
just before the call to add_wait_queue_exclusive().  The handler would
run immediately within the race window.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #32

diff --git a/module/spl/spl-taskq.c b/module/spl/spl-taskq.c
index b0677666d4698fcc1595f637179de2077719fb03..ece99aad64d8926e1aa57b62b809bac5bebe23e3 100644 (file)
--- a/module/spl/spl-taskq.c
+++ b/module/spl/spl-taskq.c
@@ -286,11 +286,10 @@ __taskq_dispatch(taskq_t *tq, task_func_t func, void *arg, uint_t flags)
        ASSERT(!(t->tqent_flags & TQENT_FLAG_PREALLOC));
 
        spin_unlock(&t->tqent_lock);
+
+       wake_up(&tq->tq_work_waitq);
 out:
        spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
-       if (rc > 0)
-               wake_up(&tq->tq_work_waitq);
-
        SRETURN(rc);
 }
 EXPORT_SYMBOL(__taskq_dispatch);
@@ -310,7 +309,6 @@ __taskq_dispatch_ent(taskq_t *tq, task_func_t func, void *arg, uint_t flags,
        /* Taskq being destroyed and all tasks drained */
        if (!(tq->tq_flags & TQ_ACTIVE)) {
                t->tqent_id = 0;
-               spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
                goto out;
        }
 
@@ -334,10 +332,10 @@ __taskq_dispatch_ent(taskq_t *tq, task_func_t func, void *arg, uint_t flags,
        t->tqent_arg = arg;
 
        spin_unlock(&t->tqent_lock);
-       spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
 
        wake_up(&tq->tq_work_waitq);
 out:
+       spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
        SEXIT;
 }
 EXPORT_SYMBOL(__taskq_dispatch_ent);
@@ -456,17 +454,17 @@ taskq_thread(void *args)
 
         while (!kthread_should_stop()) {
 
+               add_wait_queue(&tq->tq_work_waitq, &wait);
                if (list_empty(&tq->tq_pend_list) &&
                    list_empty(&tq->tq_prio_list)) {
                        spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
-                       add_wait_queue_exclusive(&tq->tq_work_waitq, &wait);
                        schedule();
-                       remove_wait_queue(&tq->tq_work_waitq, &wait);
                        spin_lock_irqsave(&tq->tq_lock, tq->tq_lock_flags);
                } else {
                        __set_current_state(TASK_RUNNING);
                }
 
+               remove_wait_queue(&tq->tq_work_waitq, &wait);
 
                if (!list_empty(&tq->tq_prio_list))
                        pend_list = &tq->tq_prio_list;