dm thin: grab a virtual cell before looking up the mapping

commit c822ed967cba38505713d59ed40a114386ef6c01 upstream.

Avoids normal IO racing with discard.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index f8c36d30eca81e7a7b67df023e0bc74f07379eed..0396d7fc1d8b22d506ca2b2edc89215f4dbaf64f 100644 (file)
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -1504,6 +1504,14 @@ static int thin_bio_map(struct dm_target *ti, struct bio *bio)
                return DM_MAPIO_SUBMITTED;
        }
 
+       /*
+        * We must hold the virtual cell before doing the lookup, otherwise
+        * there's a race with discard.
+        */
+       build_virtual_key(tc->td, block, &key);
+       if (dm_bio_detain(tc->pool->prison, &key, bio, &cell1, &cell_result))
+               return DM_MAPIO_SUBMITTED;
+
        r = dm_thin_find_block(td, block, 0, &result);
 
        /*
@@ -1527,13 +1535,10 @@ static int thin_bio_map(struct dm_target *ti, struct bio *bio)
                         * shared flag will be set in their case.
                         */
                        thin_defer_bio(tc, bio);
+                       cell_defer_no_holder_no_free(tc, &cell1);
                        return DM_MAPIO_SUBMITTED;
                }
 
-               build_virtual_key(tc->td, block, &key);
-               if (dm_bio_detain(tc->pool->prison, &key, bio, &cell1, &cell_result))
-                       return DM_MAPIO_SUBMITTED;
-
                build_data_key(tc->td, result.block, &key);
                if (dm_bio_detain(tc->pool->prison, &key, bio, &cell2, &cell_result)) {
                        cell_defer_no_holder_no_free(tc, &cell1);
@@ -1554,6 +1559,7 @@ static int thin_bio_map(struct dm_target *ti, struct bio *bio)
                         * of doing so.  Just error it.
                         */
                        bio_io_error(bio);
+                       cell_defer_no_holder_no_free(tc, &cell1);
                        return DM_MAPIO_SUBMITTED;
                }
                /* fall through */
@@ -1564,6 +1570,7 @@ static int thin_bio_map(struct dm_target *ti, struct bio *bio)
                 * provide the hint to load the metadata into cache.
                 */
                thin_defer_bio(tc, bio);
+               cell_defer_no_holder_no_free(tc, &cell1);
                return DM_MAPIO_SUBMITTED;
 
        default:
@@ -1573,6 +1580,7 @@ static int thin_bio_map(struct dm_target *ti, struct bio *bio)
                 * pool is switched to fail-io mode.
                 */
                bio_io_error(bio);
+               cell_defer_no_holder_no_free(tc, &cell1);
                return DM_MAPIO_SUBMITTED;
        }
 }