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>

diff --git a/block/blk-core.c b/block/blk-core.c
index 90f22cc..eeaca09 100644 (file)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -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);
 }