dm thin: replace dm_cell_release_singleton with cell_defer_except

Change existing users of the function dm_cell_release_singleton to share
cell_defer_except instead, and then remove the now-unused function.

Everywhere that calls dm_cell_release_singleton, the bio in question
is the holder of the cell.

If there are no non-holder entries in the cell then cell_defer_except
behaves exactly like dm_cell_release_singleton.  Conversely, if there
*are* non-holder entries then dm_cell_release_singleton must not be used
because those entries would need to be deferred.

Consequently, it is safe to replace use of dm_cell_release_singleton
with cell_defer_except.

This patch is a pre-requisite for "dm thin: fix race between
simultaneous io and discards to same block".

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-bio-prison.c

@@ -207,31 +207,6 @@ void dm_cell_release(struct dm_bio_prison_cell *cell, struct bio_list *bios)
 }
 EXPORT_SYMBOL_GPL(dm_cell_release);
 
-/*
- * There are a couple of places where we put a bio into a cell briefly
- * before taking it out again.  In these situations we know that no other
- * bio may be in the cell.  This function releases the cell, and also does
- * a sanity check.
- */
-static void __cell_release_singleton(struct dm_bio_prison_cell *cell, struct bio *bio)
-{
-	BUG_ON(cell->holder != bio);
-	BUG_ON(!bio_list_empty(&cell->bios));
-
-	__cell_release(cell, NULL);
-}
-
-void dm_cell_release_singleton(struct dm_bio_prison_cell *cell, struct bio *bio)
-{
-	unsigned long flags;
-	struct dm_bio_prison *prison = cell->prison;
-
-	spin_lock_irqsave(&prison->lock, flags);
-	__cell_release_singleton(cell, bio);
-	spin_unlock_irqrestore(&prison->lock, flags);
-}
-EXPORT_SYMBOL_GPL(dm_cell_release_singleton);
-
 /*
  * Sometimes we don't want the holder, just the additional bios.
  */

drivers/md/dm-bio-prison.h

@@ -44,7 +44,6 @@ int dm_bio_detain(struct dm_bio_prison *prison, struct dm_cell_key *key,
 		  struct bio *inmate, struct dm_bio_prison_cell **ref);
 
 void dm_cell_release(struct dm_bio_prison_cell *cell, struct bio_list *bios);
-void dm_cell_release_singleton(struct dm_bio_prison_cell *cell, struct bio *bio); // FIXME: bio arg not needed
 void dm_cell_release_no_holder(struct dm_bio_prison_cell *cell, struct bio_list *inmates);
 void dm_cell_error(struct dm_bio_prison_cell *cell);
 

drivers/md/dm-thin.c

@@ -513,8 +513,7 @@ static void cell_defer(struct thin_c *tc, struct dm_bio_prison_cell *cell,
 }
 
 /*
- * Same as cell_defer above, except it omits one particular detainee,
- * a write bio that covers the block and has already been processed.
+ * Same as cell_defer except it omits the original holder of the cell.
  */
 static void cell_defer_except(struct thin_c *tc, struct dm_bio_prison_cell *cell)
 {
@@ -936,7 +935,7 @@ static void process_discard(struct thin_c *tc, struct bio *bio)
 		 */
 		build_data_key(tc->td, lookup_result.block, &key2);
 		if (dm_bio_detain(tc->pool->prison, &key2, bio, &cell2)) {
-			dm_cell_release_singleton(cell, bio);
+			cell_defer_except(tc, cell);
 			break;
 		}
 
@@ -967,8 +966,8 @@ static void process_discard(struct thin_c *tc, struct bio *bio)
 			 * a block boundary.  So we submit the discard of a
 			 * partial block appropriately.
 			 */
-			dm_cell_release_singleton(cell, bio);
-			dm_cell_release_singleton(cell2, bio);
+			cell_defer_except(tc, cell);
+			cell_defer_except(tc, cell2);
 			if ((!lookup_result.shared) && pool->pf.discard_passdown)
 				remap_and_issue(tc, bio, lookup_result.block);
 			else
@@ -980,13 +979,13 @@ static void process_discard(struct thin_c *tc, struct bio *bio)
 		/*
 		 * It isn't provisioned, just forget it.
 		 */
-		dm_cell_release_singleton(cell, bio);
+		cell_defer_except(tc, cell);
 		bio_endio(bio, 0);
 		break;
 
 	default:
 		DMERR("discard: find block unexpectedly returned %d", r);
-		dm_cell_release_singleton(cell, bio);
+		cell_defer_except(tc, cell);
 		bio_io_error(bio);
 		break;
 	}
@@ -1041,7 +1040,7 @@ static void process_shared_bio(struct thin_c *tc, struct bio *bio,
 
 		h->shared_read_entry = dm_deferred_entry_inc(pool->shared_read_ds);
 
-		dm_cell_release_singleton(cell, bio);
+		cell_defer_except(tc, cell);
 		remap_and_issue(tc, bio, lookup_result->block);
 	}
 }
@@ -1056,7 +1055,7 @@ static void provision_block(struct thin_c *tc, struct bio *bio, dm_block_t block
 	 * Remap empty bios (flushes) immediately, without provisioning.
 	 */
 	if (!bio->bi_size) {
-		dm_cell_release_singleton(cell, bio);
+		cell_defer_except(tc, cell);
 		remap_and_issue(tc, bio, 0);
 		return;
 	}
@@ -1066,7 +1065,7 @@ static void provision_block(struct thin_c *tc, struct bio *bio, dm_block_t block
 	 */
 	if (bio_data_dir(bio) == READ) {
 		zero_fill_bio(bio);
-		dm_cell_release_singleton(cell, bio);
+		cell_defer_except(tc, cell);
 		bio_endio(bio, 0);
 		return;
 	}
@@ -1120,7 +1119,7 @@ static void process_bio(struct thin_c *tc, struct bio *bio)
 		 * TODO: this will probably have to change when discard goes
 		 * back in.
 		 */
-		dm_cell_release_singleton(cell, bio);
+		cell_defer_except(tc, cell);
 
 		if (lookup_result.shared)
 			process_shared_bio(tc, bio, block, &lookup_result);
@@ -1130,7 +1129,7 @@ static void process_bio(struct thin_c *tc, struct bio *bio)
 
 	case -ENODATA:
 		if (bio_data_dir(bio) == READ && tc->origin_dev) {
-			dm_cell_release_singleton(cell, bio);
+			cell_defer_except(tc, cell);
 			remap_to_origin_and_issue(tc, bio);
 		} else
 			provision_block(tc, bio, block, cell);
@@ -1138,7 +1137,7 @@ static void process_bio(struct thin_c *tc, struct bio *bio)
 
 	default:
 		DMERR("dm_thin_find_block() failed, error = %d", r);
-		dm_cell_release_singleton(cell, bio);
+		cell_defer_except(tc, cell);
 		bio_io_error(bio);
 		break;
 	}