[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>

2 files changed, 22 insertions, 3 deletions

diff --git a/block/elevator.c b/block/elevator.c
index 29825792cbd5..8768a367fdde 100644
--- a/block/elevator.c
+++ b/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;
 
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index e5041a02e21f..eac48bec1479 100644
--- a/block/ll_rw_blk.c
+++ b/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);