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.
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Notes:
svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=283551
@ -323,17 +323,16 @@ taskqueue_drain_tq_queue(struct taskqueue *queue)
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return;
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/*
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* Enqueue our barrier with the lowest possible priority
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* so we are inserted after all current tasks.
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* Enqueue our barrier after all current tasks, but with
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* the highest priority so that newly queued tasks cannot
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* pass it. Because of the high priority, we can not use
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* taskqueue_enqueue_locked directly (which drops the lock
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* anyway) so just insert it at tail while we have the
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* queue lock.
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*/
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TASK_INIT(&t_barrier, 0, taskqueue_task_nop_fn, &t_barrier);
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taskqueue_enqueue_locked(queue, &t_barrier);
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/*
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* Raise the barrier's priority so newly queued tasks cannot
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* pass it.
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*/
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t_barrier.ta_priority = USHRT_MAX;
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TASK_INIT(&t_barrier, USHRT_MAX, taskqueue_task_nop_fn, &t_barrier);
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STAILQ_INSERT_TAIL(&queue->tq_queue, &t_barrier, ta_link);
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t_barrier.ta_pending = 1;
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/*
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* Once the barrier has executed, all previously queued tasks
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