dm integrity: fix a deadlock due to offloading to an incorrect workqueue

If we need to perform synchronous I/O in dm_integrity_map_continue(),
we must make sure that we are not in the map function - in order to
avoid the deadlock due to bio queuing in generic_make_request. To
avoid the deadlock, we offload the request to metadata_wq.

However, metadata_wq also processes metadata updates for write requests.
If there are too many requests that get offloaded to metadata_wq at the
beginning of dm_integrity_map_continue, the workqueue metadata_wq
becomes clogged and the system is incapable of processing any metadata
updates.

This causes a deadlock because all the requests that need to do metadata
updates wait for metadata_wq to proceed and metadata_wq waits inside
wait_and_add_new_range until some existing request releases its range
lock (which doesn't happen because the range lock is released after
metadata update).

In order to fix the deadlock, we create a new workqueue offload_wq and
offload requests to it - so that processing of offload_wq is independent
from processing of metadata_wq.

Fixes: 7eada909 ("dm: add integrity target")
Cc: stable@vger.kernel.org # v4.12+
Reported-by: Heinz Mauelshagen <heinzm@redhat.com>
Tested-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-integrity.c

@@ -212,6 +212,7 @@ struct dm_integrity_c {
 	struct list_head wait_list;
 	wait_queue_head_t endio_wait;
 	struct workqueue_struct *wait_wq;
+	struct workqueue_struct *offload_wq;
 
 	unsigned char commit_seq;
 	commit_id_t commit_ids[N_COMMIT_IDS];
@@ -1439,7 +1440,7 @@ static void dec_in_flight(struct dm_integrity_io *dio)
 			dio->range.logical_sector += dio->range.n_sectors;
 			bio_advance(bio, dio->range.n_sectors << SECTOR_SHIFT);
 			INIT_WORK(&dio->work, integrity_bio_wait);
-			queue_work(ic->wait_wq, &dio->work);
+			queue_work(ic->offload_wq, &dio->work);
 			return;
 		}
 		do_endio_flush(ic, dio);
@@ -1865,7 +1866,7 @@ static void dm_integrity_map_continue(struct dm_integrity_io *dio, bool from_map
 
 	if (need_sync_io && from_map) {
 		INIT_WORK(&dio->work, integrity_bio_wait);
-		queue_work(ic->metadata_wq, &dio->work);
+		queue_work(ic->offload_wq, &dio->work);
 		return;
 	}
 
@@ -2501,7 +2502,7 @@ static void bitmap_block_work(struct work_struct *w)
 				    dio->range.n_sectors, BITMAP_OP_TEST_ALL_SET)) {
 			remove_range(ic, &dio->range);
 			INIT_WORK(&dio->work, integrity_bio_wait);
-			queue_work(ic->wait_wq, &dio->work);
+			queue_work(ic->offload_wq, &dio->work);
 		} else {
 			block_bitmap_op(ic, ic->journal, dio->range.logical_sector,
 					dio->range.n_sectors, BITMAP_OP_SET);
@@ -2524,7 +2525,7 @@ static void bitmap_block_work(struct work_struct *w)
 
 		remove_range(ic, &dio->range);
 		INIT_WORK(&dio->work, integrity_bio_wait);
-		queue_work(ic->wait_wq, &dio->work);
+		queue_work(ic->offload_wq, &dio->work);
 	}
 
 	queue_delayed_work(ic->commit_wq, &ic->bitmap_flush_work, ic->bitmap_flush_interval);
@@ -3843,6 +3844,14 @@ static int dm_integrity_ctr(struct dm_target *ti, unsigned argc, char **argv)
 		goto bad;
 	}
 
+	ic->offload_wq = alloc_workqueue("dm-integrity-offload", WQ_MEM_RECLAIM,
+					  METADATA_WORKQUEUE_MAX_ACTIVE);
+	if (!ic->offload_wq) {
+		ti->error = "Cannot allocate workqueue";
+		r = -ENOMEM;
+		goto bad;
+	}
+
 	ic->commit_wq = alloc_workqueue("dm-integrity-commit", WQ_MEM_RECLAIM, 1);
 	if (!ic->commit_wq) {
 		ti->error = "Cannot allocate workqueue";
@@ -4147,6 +4156,8 @@ static void dm_integrity_dtr(struct dm_target *ti)
 		destroy_workqueue(ic->metadata_wq);
 	if (ic->wait_wq)
 		destroy_workqueue(ic->wait_wq);
+	if (ic->offload_wq)
+		destroy_workqueue(ic->offload_wq);
 	if (ic->commit_wq)
 		destroy_workqueue(ic->commit_wq);
 	if (ic->writer_wq)