[BLOCK] limit request_fn recursion

Don't recurse back into the driver even if the unplug threshold is met,
when the driver asks for a requeue. This is both silly from a logical
point of view (requeues typically happen due to driver/hardware
shortage), and also dangerous since we could hit an endless request_fn
-> requeue -> unplug -> request_fn loop and crash on stack overrun.

Also limit blk_run_queue() to one level of recursion, similar to how
blk_start_queue() works.

This patch fixed a real problem with SLES10 and lpfc, and it could hit
any SCSI lld that returns non-zero from it's ->queuecommand() handler.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

block/elevator.c

@@ -333,6 +333,7 @@ void elv_insert(request_queue_t *q, struct request *rq, int where)
 {
 	struct list_head *pos;
 	unsigned ordseq;
+	int unplug_it = 1;
 
 	blk_add_trace_rq(q, rq, BLK_TA_INSERT);
 
@@ -399,6 +400,11 @@ void elv_insert(request_queue_t *q, struct request *rq, int where)
 		}
 
 		list_add_tail(&rq->queuelist, pos);
+		/*
+		 * most requeues happen because of a busy condition, don't
+		 * force unplug of the queue for that case.
+		 */
+		unplug_it = 0;
 		break;
 
 	default:
@@ -407,7 +413,7 @@ void elv_insert(request_queue_t *q, struct request *rq, int where)
 		BUG();
 	}
 
-	if (blk_queue_plugged(q)) {
+	if (unplug_it && blk_queue_plugged(q)) {
 		int nrq = q->rq.count[READ] + q->rq.count[WRITE]
 			- q->in_flight;
 

block/ll_rw_blk.c

@@ -1732,8 +1732,21 @@ void blk_run_queue(struct request_queue *q)
 
 	spin_lock_irqsave(q->queue_lock, flags);
 	blk_remove_plug(q);
-	if (!elv_queue_empty(q))
-		q->request_fn(q);
+
+	/*
+	 * Only recurse once to avoid overrunning the stack, let the unplug
+	 * handling reinvoke the handler shortly if we already got there.
+	 */
+	if (!elv_queue_empty(q)) {
+		if (!test_and_set_bit(QUEUE_FLAG_REENTER, &q->queue_flags)) {
+			q->request_fn(q);
+			clear_bit(QUEUE_FLAG_REENTER, &q->queue_flags);
+		} else {
+			blk_plug_device(q);
+			kblockd_schedule_work(&q->unplug_work);
+		}
+	}
+
 	spin_unlock_irqrestore(q->queue_lock, flags);
 }
 EXPORT_SYMBOL(blk_run_queue);