Prevent live-lock and access of destroyed data in taskqueue_drain_all().

Phabric:	https://reviews.freebsd.org/D1247
Reviewed by:	jhb, avg
Sponsored by:	Spectra Logic Corporation

sys/kern_subr_taskqueue.c:
	Modify taskqueue_drain_all() processing to use a temporary
	"barrier task", rather than rely on a user task that may
	be destroyed during taskqueue_drain_all()'s execution.  The
	barrier task is queued behind all previously queued tasks
	and then has its priority elevated so that future tasks
	cannot pass it in the queue.

	Use a similar barrier scheme to drain threads processing
	current tasks.  This requires taskqueue_run_locked() to
	insert and remove the taskqueue_busy object for the running
	thread for every task processed.

share/man/man9/taskqueue.9:
	Remove warning about live-lock issues with taskqueue_drain_all()
	and indicate that it does not wait for tasks queued after
	it begins processing.

1 files changed, 10 insertions, 4 deletions

diff --git a/share/man/man9/taskqueue.9 b/share/man/man9/taskqueue.9
index 662a9d5..e2b105f 100644
--- a/share/man/man9/taskqueue.9
+++ b/share/man/man9/taskqueue.9
@@ -28,7 +28,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 24, 2014
+.Dd January 4, 2015
 .Dt TASKQUEUE 9
 .Os
 .Sh NAME
@@ -285,10 +285,16 @@ The
 .Fn taskqueue_drain_all
 function is used to wait for all pending and running tasks that
 are enqueued on the taskqueue to finish.
-The caller must arrange that the tasks are not re-enqueued.
-Note that
+Tasks posted to the taskqueue after
 .Fn taskqueue_drain_all
-currently does not handle tasks with delayed enqueueing.
+begins processing,
+including pending enqueues scheduled by a previous call to 
+.Fn taskqueue_enqueue_timeout ,
+do not extend the wait time of
+.Fn taskqueue_drain_all
+and may complete after 
+.Fn taskqueue_drain_all
+returns.
 .Pp
 The
 .Fn taskqueue_block