Commit b10ebd34 authored by Joe Thornber's avatar Joe Thornber Committed by Mike Snitzer
Browse files

dm thin: fix rcu_read_lock being held in code that can sleep

Commit c140e1c4

 ("dm thin: use per thin device deferred bio lists")
introduced the use of an rculist for all active thin devices.  The use
of rcu_read_lock() in process_deferred_bios() can result in a BUG if a
dm_bio_prison_cell must be allocated as a side-effect of bio_detain():

 BUG: sleeping function called from invalid context at mm/mempool.c:203
 in_atomic(): 1, irqs_disabled(): 0, pid: 6, name: kworker/u8:0
 3 locks held by kworker/u8:0/6:
   #0:  ("dm-" "thin"){.+.+..}, at: [<ffffffff8106be42>] process_one_work+0x192/0x550
   #1:  ((&pool->worker)){+.+...}, at: [<ffffffff8106be42>] process_one_work+0x192/0x550
   #2:  (rcu_read_lock){.+.+..}, at: [<ffffffff816360b5>] do_worker+0x5/0x4d0

We can't process deferred bios with the rcu lock held, since
dm_bio_prison_cell allocation may block if the bio-prison's cell mempool
is exhausted.

To fix:

- Introduce a refcount and completion field to each thin_c

- Add thin_get/put methods for adjusting the refcount.  If the refcount
  hits zero then the completion is triggered.

- Initialise refcount to 1 when creating thin_c

- When iterating the active_thins list we thin_get() whilst the rcu
  lock is held.

- After the rcu lock is dropped we process the deferred bios for that

- When destroying a thin_c we thin_put() and then wait for the
  completion -- to avoid a race between the worker thread iterating
  from that thin_c and destroying the thin_c.
Signed-off-by: default avatarJoe Thornber <>
Signed-off-by: default avatarMike Snitzer <>
parent 5e3283e2
......@@ -232,6 +232,13 @@ struct thin_c {
struct bio_list deferred_bio_list;
struct bio_list retry_on_resume_list;
struct rb_root sort_bio_list; /* sorted list of deferred bios */
* Ensures the thin is not destroyed until the worker has finished
* iterating the active_thins list.
atomic_t refcount;
struct completion can_destroy;
......@@ -1486,6 +1493,45 @@ static void process_thin_deferred_bios(struct thin_c *tc)
static void thin_get(struct thin_c *tc);
static void thin_put(struct thin_c *tc);
* We can't hold rcu_read_lock() around code that can block. So we
* find a thin with the rcu lock held; bump a refcount; then drop
* the lock.
static struct thin_c *get_first_thin(struct pool *pool)
struct thin_c *tc = NULL;
if (!list_empty(&pool->active_thins)) {
tc = list_entry_rcu(pool->, struct thin_c, list);
return tc;
static struct thin_c *get_next_thin(struct pool *pool, struct thin_c *tc)
struct thin_c *old_tc = tc;
list_for_each_entry_continue_rcu(tc, &pool->active_thins, list) {
return tc;
return NULL;
static void process_deferred_bios(struct pool *pool)
unsigned long flags;
......@@ -1493,10 +1539,11 @@ static void process_deferred_bios(struct pool *pool)
struct bio_list bios;
struct thin_c *tc;
list_for_each_entry_rcu(tc, &pool->active_thins, list)
tc = get_first_thin(pool);
while (tc) {
tc = get_next_thin(pool, tc);
* If there are any deferred flush bios, we must commit
......@@ -3061,11 +3108,25 @@ static struct target_type pool_target = {
* Thin target methods
static void thin_get(struct thin_c *tc)
static void thin_put(struct thin_c *tc)
if (atomic_dec_and_test(&tc->refcount))
static void thin_dtr(struct dm_target *ti)
struct thin_c *tc = ti->private;
unsigned long flags;
spin_lock_irqsave(&tc->pool->lock, flags);
spin_unlock_irqrestore(&tc->pool->lock, flags);
......@@ -3192,6 +3253,9 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
atomic_set(&tc->refcount, 1);
spin_lock_irqsave(&tc->pool->lock, flags);
list_add_tail_rcu(&tc->list, &tc->pool->active_thins);
spin_unlock_irqrestore(&tc->pool->lock, flags);
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