Commit 9eef87da authored Feb 16, 2010 by Kiyoshi Ueda Committed by Alasdair G Kergon Feb 16, 2010

dm mpath: fix stall when requeueing io



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>
Cc: stable@kernel.org
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

parent 558569aa

drivers/md/dm.c

+17 −4

Original line number	Diff line number	Diff line
		@@ -1595,10 +1595,15 @@ static int dm_prep_fn(struct request_queue q, struct request rq)
		return BLKPREP_OK;
		}

		static void map_request(struct dm_target ti, struct request clone,
		/*
		* 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;
		int r, requeued = 0;
		struct dm_rq_target_io *tio = clone->end_io_data;

		/*
		@@ -1625,6 +1630,7 @@ static void map_request(struct dm_target ti, struct request clone,
		case DM_MAPIO_REQUEUE:
		/* The target wants to requeue the I/O */
		dm_requeue_unmapped_request(clone);
		requeued = 1;
		break;
		default:
		if (r > 0) {
		@@ -1636,6 +1642,8 @@ static void map_request(struct dm_target ti, struct request clone,
		dm_kill_unmapped_request(clone, r);
		break;
		}

		return requeued;
		}

		/*
		@@ -1677,12 +1685,17 @@ static void dm_request_fn(struct request_queue *q)
		atomic_inc(&md->pending[rq_data_dir(clone)]);

		spin_unlock(q->queue_lock);
		map_request(ti, clone, md);
		if (map_request(ti, clone, 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 */

Admin message