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block: splice plug list to local context 

If the request_fn ends up blocking, we could be re-entering
the plug flush. Since the list is protected by explicitly
not allowing schedule events, this isn't a terribly good idea.

Additionally, it can cause us to recurse. As request_fn called by
__blk_run_queue is allowed to 'schedule()' (after dropping the queue
lock of course), it is possible to get a recursive call:

 schedule -> blk_flush_plug -> __blk_finish_plug -> flush_plug_list
      -> __blk_run_queue -> request_fn -> schedule

We must make sure that the second schedule does not call into
blk_flush_plug again.  So instead of leaving the list of requests on
blk_plug->list, move them to a separate list leaving blk_plug->list
empty.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
This commit is contained in:
NeilBrown 2011-04-11 14:13:10 +02:00 committed by Jens Axboe
parent 4263a2f1da
commit 109b81296c
1 changed files with 9 additions and 5 deletions
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14
block/blk-core.c
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@ -2673,19 +2673,24 @@ static void flush_plug_list(struct blk_plug *plug)
struct request_queue *q;
unsigned long flags;
struct request *rq;
LIST_HEAD(list);
BUG_ON(plug->magic != PLUG_MAGIC);
if (list_empty(&plug->list))
return;
if (plug->should_sort)
list_sort(NULL, &plug->list, plug_rq_cmp);
list_splice_init(&plug->list, &list);
if (plug->should_sort) {
list_sort(NULL, &list, plug_rq_cmp);
plug->should_sort = 0;
}
q = NULL;
local_irq_save(flags);
while (!list_empty(&plug->list)) {
rq = list_entry_rq(plug->list.next);
while (!list_empty(&list)) {
rq = list_entry_rq(list.next);
list_del_init(&rq->queuelist);
BUG_ON(!(rq->cmd_flags & REQ_ON_PLUG));
BUG_ON(!rq->q);
@ -2713,7 +2718,6 @@ static void flush_plug_list(struct blk_plug *plug)
spin_unlock(q->queue_lock);
}
BUG_ON(!list_empty(&plug->list));
local_irq_restore(flags);
}