1 files changed, 12 insertions, 4 deletions

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index 4843801173fe..0f86d802b533 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -1936,6 +1936,14 @@ 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);
 
         /*
@@ -1959,13 +1967,10 @@ 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);
@@ -1986,6 +1991,7 @@ static int thin_bio_map(struct dm_target *ti, struct bio *bio)
                          * of doing so.
                          */
                         handle_unserviceable_bio(tc->pool, bio);
+                        cell_defer_no_holder_no_free(tc, &cell1);
                         return DM_MAPIO_SUBMITTED;
                 }
                 /* fall through */
@@ -1996,6 +2002,7 @@ 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:
@@ -2005,6 +2012,7 @@ 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;
         }
 }