From patchwork Wed Feb 3 07:03:11 2010 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: rqdm: fix stall when ->map_rq returns DM_MAPIO_REQUEUE Date: Wed, 03 Feb 2010 07:03:11 -0000 From: Kiyoshi Ueda X-Patchwork-Id: 76618 Message-Id: <4B691FAF.6030509@ct.jp.nec.com> To: Alasdair Kergon Cc: device-mapper development Hi Alasdair, 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. It is for 2.6.33-rc6, and no dependency on your editing patches. Please review and apply. 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 Signed-off-by: Jun'ichi Nomura Cc: Alasdair G Kergon --- drivers/md/dm.c | 21 +++++++++++++++++---- drivers/md/dm.c | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-) -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel Index: linux-2.6.33-rc8/drivers/md/dm.c =================================================================== --- linux-2.6.33-rc8.orig/drivers/md/dm.c +++ linux-2.6.33-rc8/drivers/md/dm.c @@ -1595,10 +1595,15 @@ static int dm_prep_fn(struct request_que return BLKPREP_OK; } -static void map_request(struct dm_target *ti, struct request *clone, - 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; + 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 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 dm_kill_unmapped_request(clone, r); break; } + + return requeued; } /* @@ -1677,12 +1685,17 @@ static void dm_request_fn(struct request 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 */