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Subject[93/93] dm mpath: fix stall when requeueing io
2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>

commit 9eef87da2a8ea4920e0d913ff977cac064b68ee0 upstream.

This patch fixes the problem that system may stall if target's ->map_rq
returns DM_MAPIO_REQUEUE in map_request().
E.g. stall happens on 1 CPU box when a dm-mpath device with queue_if_no_path
bounces between all-paths-down and paths-up on I/O load.

When target's ->map_rq returns DM_MAPIO_REQUEUE, map_request() requeues
the request and returns to dm_request_fn(). Then, dm_request_fn()
doesn't exit the I/O dispatching loop and continues processing
the requeued request again.
This map and requeue loop can be done with interrupt disabled,
so 1 CPU system can be stalled if this situation happens.

For example, commands below can stall my 1 CPU box within 1 minute or so:
# dmsetup table mp
mp: 0 2097152 multipath 1 queue_if_no_path 0 1 1 service-time 0 1 2 8:144 1 1
# while true; do dd if=/dev/mapper/mp of=/dev/null bs=1M count=100; done &
# while true; do \
> dmsetup message mp 0 "fail_path 8:144" \
> dmsetup suspend --noflush mp \
> dmsetup resume mp \
> dmsetup message mp 0 "reinstate_path 8:144" \
> done

To fix the problem above, this patch changes dm_request_fn() to exit
the I/O dispatching loop once if a request is requeued in map_request().

Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
drivers/md/dm.c | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)

--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1487,11 +1487,15 @@ static int dm_prep_fn(struct request_que
return BLKPREP_OK;
}

-static void map_request(struct dm_target *ti, struct request *rq,
- struct mapped_device *md)
+/*
+ * Returns:
+ * 0 : the request has been processed (not requeued)
+ * !0 : the request has been requeued
+ */
+static int map_request(struct dm_target *ti, struct request *clone,
+ struct mapped_device *md)
{
- int r;
- struct request *clone = rq->special;
+ int r, requeued = 0;
struct dm_rq_target_io *tio = clone->end_io_data;

/*
@@ -1516,6 +1520,7 @@ static void map_request(struct dm_target
case DM_MAPIO_REQUEUE:
/* The target wants to requeue the I/O */
dm_requeue_unmapped_request(clone);
+ requeued = 1;
break;
default:
if (r > 0) {
@@ -1527,6 +1532,8 @@ static void map_request(struct dm_target
dm_kill_unmapped_request(clone, r);
break;
}
+
+ return requeued;
}

/*
@@ -1568,12 +1575,16 @@ static void dm_request_fn(struct request

blk_start_request(rq);
spin_unlock(q->queue_lock);
- map_request(ti, rq, md);
+ if (map_request(ti, rq, md))
+ goto requeued;
spin_lock_irq(q->queue_lock);
}

goto out;

+requeued:
+ spin_lock_irq(q->queue_lock);
+
plug_and_out:
if (!elv_queue_empty(q))
/* Some requests still remain, retry later */



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