Fix taskq_wait_id()

The existing taskq_wait_id() function can incorrectly block
indefinitely.  Reimplement it more simply using wait_event()
in a similar fashion to taskq_wait_all().

This flaw was uncovered in the context of moving vn_rdwr() to
a taskq.  Previously taskq_wait_id() had no consumers outside
the SPLAT task framework which is why the issue went unnoticed.

Signed-off-by: Brian Behlendorf <[email protected]>

1 files changed, 14 insertions, 26 deletions

diff --git a/module/spl/spl-taskq.c b/module/spl/spl-taskq.c
index 4feca0452..bcdc98f97 100644
--- a/module/spl/spl-taskq.c
+++ b/module/spl/spl-taskq.c
@@ -342,39 +342,27 @@ taskq_find(taskq_t *tq, taskqid_t id, int *active)
 	SRETURN(NULL);
 }
 
-/*
- * The taskq_wait_id() function blocks until the passed task id completes.
- * This does not guarantee that all lower task id's have completed.
- */
-void
-taskq_wait_id(taskq_t *tq, taskqid_t id)
+static int
+taskq_wait_id_check(taskq_t *tq, taskqid_t id)
 {
-	DEFINE_WAIT(wait);
-	taskq_ent_t *t;
 	int active = 0;
-	SENTRY;
-
-	ASSERT(tq);
-	ASSERT(id > 0);
+	int rc;
 
 	spin_lock_irqsave(&tq->tq_lock, tq->tq_lock_flags);
-	t = taskq_find(tq, id, &active);
-	if (t)
-		prepare_to_wait(&t->tqent_waitq, &wait, TASK_UNINTERRUPTIBLE);
+	rc = (taskq_find(tq, id, &active) == NULL);
 	spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
 
-	/*
-	 * We rely on the kernels autoremove_wake_function() function to
-	 * remove us from the wait queue in the context of wake_up().
-	 * Once woken the taskq_ent_t pointer must never be accessed.
-	 */
-	if (t) {
-		t = NULL;
-		schedule();
-		__set_current_state(TASK_RUNNING);
-	}
+	return (rc);
+}
 
-	SEXIT;
+/*
+ * The taskq_wait_id() function blocks until the passed task id completes.
+ * This does not guarantee that all lower task ids have completed.
+ */
+void
+taskq_wait_id(taskq_t *tq, taskqid_t id)
+{
+	wait_event(tq->tq_wait_waitq, taskq_wait_id_check(tq, id));
 }
 EXPORT_SYMBOL(taskq_wait_id);