[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>

diff --git a/block/elevator.c b/block/elevator.c
index 2982579..8768a36 100644 (file)
--- 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 e5041a0..eac48be 100644 (file)
--- 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);