[dm-devel] [PATCH 01/14] dm-thin: don't use the bi_next field for the holder of a cell.

[Joe Thornber]

BUGFIX: theoretical issue when stacking thin on devices that defer
bios (not seen in the wild).

When a cell is created the bio that triggered creation (the holder)
was added to the same bio list as subsequent bios.  In some cases we
let the holder continue to lower devices.  If those lower devices use
the bi_next field there will be trouble ...

This change keeps the holder in a new, separate field of the cell.

This also has the effect of simplifying some code that had to work out
which bio was the holder.

Signed-off-by: Joe Thornber <ejt at redhat.com>
Signed-off-by: Mike Snitzer <snitzer at redhat.com>
---
 drivers/md/dm-thin.c |   81 ++++++++++++++++++++++++++++++++------------------
 1 files changed, 52 insertions(+), 29 deletions(-)

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index c308757..5abcc9b 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -124,7 +124,7 @@ struct cell {
 	struct hlist_node list;
 	struct bio_prison *prison;
 	struct cell_key key;
-	unsigned count;
+	struct bio *holder;
 	struct bio_list bios;
 };
 
@@ -220,8 +220,7 @@ static struct cell *__search_bucket(struct hlist_head *bucket,
  * This may block if a new cell needs allocating.  You must ensure that
  * cells will be unlocked even if the calling thread is blocked.
  *
- * Returns the number of entries in the cell prior to the new addition
- * or < 0 on failure.
+ * Returns 1 if the cell was already held, 0 if @inmate is the new holder.
  */
 static int bio_detain(struct bio_prison *prison, struct cell_key *key,
 		      struct bio *inmate, struct cell **ref)
@@ -256,21 +255,25 @@ static int bio_detain(struct bio_prison *prison, struct cell_key *key,
 
 			cell->prison = prison;
 			memcpy(&cell->key, key, sizeof(cell->key));
-			cell->count = 0;
+			cell->holder = inmate;
 			bio_list_init(&cell->bios);
 			hlist_add_head(&cell->list, prison->cells + hash);
+			r = 0;
+
+		} else {
+			mempool_free(cell2, prison->cell_pool);
+			cell2 = NULL;
+			r = 1;
+			bio_list_add(&cell->bios, inmate);
 		}
-	}
 
-	r = cell->count++;
-	bio_list_add(&cell->bios, inmate);
+	} else {
+		r = 1;
+		bio_list_add(&cell->bios, inmate);
+	}
 	spin_unlock_irqrestore(&prison->lock, flags);
 
-	if (cell2)
-		mempool_free(cell2, prison->cell_pool);
-
 	*ref = cell;
-
 	return r;
 }
 
@@ -283,8 +286,10 @@ static void __cell_release(struct cell *cell, struct bio_list *inmates)
 
 	hlist_del(&cell->list);
 
-	if (inmates)
+	if (inmates) {
+		bio_list_add(inmates, cell->holder);
 		bio_list_merge(inmates, &cell->bios);
+	}
 
 	mempool_free(cell, prison->cell_pool);
 }
@@ -305,22 +310,45 @@ static void cell_release(struct cell *cell, struct bio_list *bios)
  * bio may be in the cell.  This function releases the cell, and also does
  * a sanity check.
  */
+static void __cell_release_singleton(struct cell *cell, struct bio *bio)
+{
+	hlist_del(&cell->list);
+	BUG_ON(cell->holder != bio);
+	BUG_ON(!bio_list_empty(&cell->bios));
+}
+
 static void cell_release_singleton(struct cell *cell, struct bio *bio)
 {
-	struct bio_prison *prison = cell->prison;
-	struct bio_list bios;
-	struct bio *b;
 	unsigned long flags;
-
-	bio_list_init(&bios);
+	struct bio_prison *prison = cell->prison;
 
 	spin_lock_irqsave(&prison->lock, flags);
-	__cell_release(cell, &bios);
+	__cell_release_singleton(cell, bio);
 	spin_unlock_irqrestore(&prison->lock, flags);
+}
 
-	b = bio_list_pop(&bios);
-	BUG_ON(b != bio);
-	BUG_ON(!bio_list_empty(&bios));
+/*
+ * Sometimes we don't want the holder, just the additional bios.
+ */
+static void __cell_release_no_holder(struct cell *cell, struct bio_list *inmates)
+{
+	struct bio_prison *prison = cell->prison;
+
+	hlist_del(&cell->list);
+	if (inmates)
+		bio_list_merge(inmates, &cell->bios);
+
+	mempool_free(cell, prison->cell_pool);
+}
+
+static void cell_release_no_holder(struct cell *cell, struct bio_list *inmates)
+{
+	unsigned long flags;
+	struct bio_prison *prison = cell->prison;
+
+	spin_lock_irqsave(&prison->lock, flags);
+	__cell_release_no_holder(cell, inmates);
+	spin_unlock_irqrestore(&prison->lock, flags);
 }
 
 static void cell_error(struct cell *cell)
@@ -800,21 +828,16 @@ static void cell_defer(struct thin_c *tc, struct 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.
  */
-static void cell_defer_except(struct thin_c *tc, struct cell *cell,
-			      struct bio *exception)
+static void cell_defer_except(struct thin_c *tc, struct cell *cell)
 {
 	struct bio_list bios;
-	struct bio *bio;
 	struct pool *pool = tc->pool;
 	unsigned long flags;
 
 	bio_list_init(&bios);
-	cell_release(cell, &bios);
 
 	spin_lock_irqsave(&pool->lock, flags);
-	while ((bio = bio_list_pop(&bios)))
-		if (bio != exception)
-			bio_list_add(&pool->deferred_bios, bio);
+	cell_release_no_holder(cell, &pool->deferred_bios);
 	spin_unlock_irqrestore(&pool->lock, flags);
 
 	wake_worker(pool);
@@ -854,7 +877,7 @@ static void process_prepared_mapping(struct new_mapping *m)
 	 * the bios in the cell.
 	 */
 	if (bio) {
-		cell_defer_except(tc, m->cell, bio);
+		cell_defer_except(tc, m->cell);
 		bio_endio(bio, 0);
 	} else
 		cell_defer(tc, m->cell, m->data_block);
-- 
1.7.9.1