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

Browse Source 
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.
This commit is contained in:
Justin T. Gibbs 2015-01-04 19:55:44 +00:00
parent aad37c8145
commit 5b326a32ce
2 changed files with 98 additions and 20 deletions
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14
share/man/man9/taskqueue.9
View File

@ -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

104
sys/kern/subr_taskqueue.c
View File

@ -56,6 +56,8 @@ struct taskqueue_busy {
TAILQ_ENTRY(taskqueue_busy) tb_link;
};
struct task * const TB_DRAIN_WAITER = (struct task *)0x1;
struct taskqueue {
STAILQ_HEAD(, task) tq_queue;
taskqueue_enqueue_fn tq_enqueue;
@ -241,6 +243,7 @@ taskqueue_enqueue_locked(struct taskqueue *queue, struct task *task)
/* Return with lock released. */
return (0);
}
int
taskqueue_enqueue(struct taskqueue *queue, struct task *task)
{
@ -302,12 +305,82 @@ taskqueue_enqueue_timeout(struct taskqueue *queue,
}
static void
taskqueue_drain_running(struct taskqueue *queue)
taskqueue_task_nop_fn(void *context, int pending)
{
}
while (!TAILQ_EMPTY(&queue->tq_active))
TQ_SLEEP(queue, &queue->tq_active, &queue->tq_mutex,
PWAIT, "-", 0);
/*
* Block until all currently queued tasks in this taskqueue
* have begun execution. Tasks queued during execution of
* this function are ignored.
*/
static void
taskqueue_drain_tq_queue(struct taskqueue *queue)
{
struct task t_barrier;
if (STAILQ_EMPTY(&queue->tq_queue))
return;
/*
* Enqueue our barrier with the lowest possible priority
* so we are inserted after all current tasks.
*/
TASK_INIT(&t_barrier, 0, taskqueue_task_nop_fn, &t_barrier);
taskqueue_enqueue_locked(queue, &t_barrier);
/*
* Raise the barrier's priority so newly queued tasks cannot
* pass it.
*/
t_barrier.ta_priority = USHRT_MAX;
/*
* Once the barrier has executed, all previously queued tasks
* have completed or are currently executing.
*/
while (t_barrier.ta_pending != 0)
TQ_SLEEP(queue, &t_barrier, &queue->tq_mutex, PWAIT, "-", 0);
}
/*
* Block until all currently executing tasks for this taskqueue
* complete. Tasks that begin execution during the execution
* of this function are ignored.
*/
static void
taskqueue_drain_tq_active(struct taskqueue *queue)
{
struct taskqueue_busy tb_marker, *tb_first;
if (TAILQ_EMPTY(&queue->tq_active))
return;
/* Block taskq_terminate().*/
queue->tq_callouts++;
/*
* Wait for all currently executing taskqueue threads
* to go idle.
*/
tb_marker.tb_running = TB_DRAIN_WAITER;
TAILQ_INSERT_TAIL(&queue->tq_active, &tb_marker, tb_link);
while (TAILQ_FIRST(&queue->tq_active) != &tb_marker)
TQ_SLEEP(queue, &tb_marker, &queue->tq_mutex, PWAIT, "-", 0);
TAILQ_REMOVE(&queue->tq_active, &tb_marker, tb_link);
/*
* Wakeup any other drain waiter that happened to queue up
* without any intervening active thread.
*/
tb_first = TAILQ_FIRST(&queue->tq_active);
if (tb_first != NULL && tb_first->tb_running == TB_DRAIN_WAITER)
wakeup(tb_first);
/* Release taskqueue_terminate(). */
queue->tq_callouts--;
if ((queue->tq_flags & TQ_FLAGS_ACTIVE) == 0)
wakeup_one(queue->tq_threads);
}
void
@ -334,14 +407,16 @@ static void
taskqueue_run_locked(struct taskqueue *queue)
{
struct taskqueue_busy tb;
struct taskqueue_busy *tb_first;
struct task *task;
int pending;
TQ_ASSERT_LOCKED(queue);
tb.tb_running = NULL;
TAILQ_INSERT_TAIL(&queue->tq_active, &tb, tb_link);
while (STAILQ_FIRST(&queue->tq_queue)) {
TAILQ_INSERT_TAIL(&queue->tq_active, &tb, tb_link);
/*
* Carefully remove the first task from the queue and
* zero its pending count.
@ -358,10 +433,13 @@ taskqueue_run_locked(struct taskqueue *queue)
TQ_LOCK(queue);
tb.tb_running = NULL;
wakeup(task);
TAILQ_REMOVE(&queue->tq_active, &tb, tb_link);
tb_first = TAILQ_FIRST(&queue->tq_active);
if (tb_first != NULL &&
tb_first->tb_running == TB_DRAIN_WAITER)
wakeup(tb_first);
}
TAILQ_REMOVE(&queue->tq_active, &tb, tb_link);
if (TAILQ_EMPTY(&queue->tq_active))
wakeup(&queue->tq_active);
}
void
@ -448,19 +526,13 @@ taskqueue_drain(struct taskqueue *queue, struct task *task)
void
taskqueue_drain_all(struct taskqueue *queue)
{
struct task *task;
if (!queue->tq_spin)
WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL, __func__);
TQ_LOCK(queue);
task = STAILQ_LAST(&queue->tq_queue, task, ta_link);
if (task != NULL)
while (task->ta_pending != 0)
TQ_SLEEP(queue, task, &queue->tq_mutex, PWAIT, "-", 0);
taskqueue_drain_running(queue);
KASSERT(STAILQ_EMPTY(&queue->tq_queue),
("taskqueue queue is not empty after draining"));
taskqueue_drain_tq_queue(queue);
taskqueue_drain_tq_active(queue);
TQ_UNLOCK(queue);
}