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authorJoe Thornber <ejt@redhat.com>2014-04-08 11:29:01 +0100
committerMike Snitzer <snitzer@redhat.com>2014-04-08 10:18:35 -0400
commitb10ebd34cccae1b431caf1be54919aede2be7cbe (patch)
treef8f848169991555d7ea0719740da37645e405c99 /drivers/md/dm-thin.c
parent5e3283e2920a0bd8a806964d80274b8756e0dd7f (diff)
downloadop-kernel-dev-b10ebd34cccae1b431caf1be54919aede2be7cbe.zip
op-kernel-dev-b10ebd34cccae1b431caf1be54919aede2be7cbe.tar.gz
dm thin: fix rcu_read_lock being held in code that can sleep
Commit c140e1c4e23 ("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 thin. - 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: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Diffstat (limited to 'drivers/md/dm-thin.c')
-rw-r--r--drivers/md/dm-thin.c70
1 files changed, 67 insertions, 3 deletions
diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index ae5fd0b..28fc282 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -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)
blk_finish_plug(&plug);
}
+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;
+
+ rcu_read_lock();
+ if (!list_empty(&pool->active_thins)) {
+ tc = list_entry_rcu(pool->active_thins.next, struct thin_c, list);
+ thin_get(tc);
+ }
+ rcu_read_unlock();
+
+ return tc;
+}
+
+static struct thin_c *get_next_thin(struct pool *pool, struct thin_c *tc)
+{
+ struct thin_c *old_tc = tc;
+
+ rcu_read_lock();
+ list_for_each_entry_continue_rcu(tc, &pool->active_thins, list) {
+ thin_get(tc);
+ thin_put(old_tc);
+ rcu_read_unlock();
+ return tc;
+ }
+ thin_put(old_tc);
+ rcu_read_unlock();
+
+ 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;
- rcu_read_lock();
- list_for_each_entry_rcu(tc, &pool->active_thins, list)
+ tc = get_first_thin(pool);
+ while (tc) {
process_thin_deferred_bios(tc);
- rcu_read_unlock();
+ 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)
+{
+ atomic_inc(&tc->refcount);
+}
+
+static void thin_put(struct thin_c *tc)
+{
+ if (atomic_dec_and_test(&tc->refcount))
+ complete(&tc->can_destroy);
+}
+
static void thin_dtr(struct dm_target *ti)
{
struct thin_c *tc = ti->private;
unsigned long flags;
+ thin_put(tc);
+ wait_for_completion(&tc->can_destroy);
+
spin_lock_irqsave(&tc->pool->lock, flags);
list_del_rcu(&tc->list);
spin_unlock_irqrestore(&tc->pool->lock, flags);
@@ -3192,6 +3253,9 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
mutex_unlock(&dm_thin_pool_table.mutex);
+ atomic_set(&tc->refcount, 1);
+ init_completion(&tc->can_destroy);
+
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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