From eb3d0c5d8c500ab412ff5ed6e93c7a6917d9f2da Mon Sep 17 00:00:00 2001 From: Xin LI Date: Tue, 26 May 2015 01:40:33 +0000 Subject: [PATCH] MFuser/delphij/zfs-arc-rebase@r281754: In r256613, taskqueue_enqueue_locked() have been modified to release the task queue lock before returning. In r276665, taskqueue_drain_all() will call taskqueue_enqueue_locked() to insert the barrier task into the queue, but did not reacquire the lock after it but later code expects the lock still being held (e.g. TQ_SLEEP()). The barrier task is special and if we release then reacquire the lock, there would be a small race window where a high priority task could sneak into the queue. Looking more closely, the race seems to be tolerable but is undesirable from semantics standpoint. To solve this, in taskqueue_drain_tq_queue(), instead of directly calling taskqueue_enqueue_locked(), insert the barrier task directly without releasing the lock. --- sys/kern/subr_taskqueue.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-) diff --git a/sys/kern/subr_taskqueue.c b/sys/kern/subr_taskqueue.c index 2cf1f63d9a2..f0a5a1ba85e 100644 --- a/sys/kern/subr_taskqueue.c +++ b/sys/kern/subr_taskqueue.c @@ -323,17 +323,16 @@ taskqueue_drain_tq_queue(struct taskqueue *queue) return; /* - * Enqueue our barrier with the lowest possible priority - * so we are inserted after all current tasks. + * Enqueue our barrier after all current tasks, but with + * the highest priority so that newly queued tasks cannot + * pass it. Because of the high priority, we can not use + * taskqueue_enqueue_locked directly (which drops the lock + * anyway) so just insert it at tail while we have the + * queue lock. */ - 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; + TASK_INIT(&t_barrier, USHRT_MAX, taskqueue_task_nop_fn, &t_barrier); + STAILQ_INSERT_TAIL(&queue->tq_queue, &t_barrier, ta_link); + t_barrier.ta_pending = 1; /* * Once the barrier has executed, all previously queued tasks