dm snapshot: suspend merging snapshot when doing exception handover

The "dm snapshot: suspend origin when doing exception handover" commit
fixed a exception store handover bug associated with pending exceptions
to the "snapshot-origin" target.

However, a similar problem exists in snapshot merging.  When snapshot
merging is in progress, we use the target "snapshot-merge" instead of
"snapshot-origin".  Consequently, during exception store handover, we
must find the snapshot-merge target and suspend its associated
mapped_device.

To avoid lockdep warnings, the target must be suspended and resumed
without holding _origins_lock.

Introduce a dm_hold() function that grabs a reference on a
mapped_device, but unlike dm_get(), it doesn't crash if the device has
the DMF_FREEING flag set, it returns an error in this case.

In snapshot_resume() we grab the reference to the origin device using
dm_hold() while holding _origins_lock (_origins_lock guarantees that the
device won't disappear).  Then we release _origins_lock, suspend the
device and grab _origins_lock again.

NOTE to stable@ people:
When backporting to kernels 3.18 and older, use dm_internal_suspend and
dm_internal_resume instead of dm_internal_suspend_fast and
dm_internal_resume_fast.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

3 files changed, 43 insertions, 6 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index c2bf822bad6f..f83a0f3fc365 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1888,20 +1888,39 @@ static int snapshot_preresume(struct dm_target *ti)
 static void snapshot_resume(struct dm_target *ti)
 {
         struct dm_snapshot *s = ti->private;
-        struct dm_snapshot *snap_src = NULL, *snap_dest = NULL;
+        struct dm_snapshot *snap_src = NULL, *snap_dest = NULL, *snap_merging = NULL;
         struct dm_origin *o;
         struct mapped_device *origin_md = NULL;
+        bool must_restart_merging = false;
 
         down_read(&_origins_lock);
 
         o = __lookup_dm_origin(s->origin->bdev);
         if (o)
                 origin_md = dm_table_get_md(o->ti->table);
+        if (!origin_md) {
+                (void) __find_snapshots_sharing_cow(s, NULL, NULL, &snap_merging);
+                if (snap_merging)
+                        origin_md = dm_table_get_md(snap_merging->ti->table);
+        }
         if (origin_md == dm_table_get_md(ti->table))
                 origin_md = NULL;
+        if (origin_md) {
+                if (dm_hold(origin_md))
+                        origin_md = NULL;
+        }
 
-        if (origin_md)
+        up_read(&_origins_lock);
+
+        if (origin_md) {
                 dm_internal_suspend_fast(origin_md);
+                if (snap_merging && test_bit(RUNNING_MERGE, &snap_merging->state_bits)) {
+                        must_restart_merging = true;
+                        stop_merge(snap_merging);
+                }
+        }
+
+        down_read(&_origins_lock);
 
         (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
         if (snap_src && snap_dest) {
@@ -1912,11 +1931,15 @@ static void snapshot_resume(struct dm_target *ti)
                 up_write(&snap_src->lock);
         }
 
-        if (origin_md)
-                dm_internal_resume_fast(origin_md);
-
         up_read(&_origins_lock);
 
+        if (origin_md) {
+                if (must_restart_merging)
+                        start_merge(snap_merging);
+                dm_internal_resume_fast(origin_md);
+                dm_put(origin_md);
+        }
+
         /* Now we have correct chunk size, reregister */
         reregister_snapshot(s);
 
@@ -2360,7 +2383,7 @@ static struct target_type snapshot_target = {
 
 static struct target_type merge_target = {
         .name    = dm_snapshot_merge_target_name,
-        .version = {1, 2, 0},
+        .version = {1, 3, 0},
         .module  = THIS_MODULE,
         .ctr     = snapshot_ctr,
         .dtr     = snapshot_dtr,
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 6e2b2e97abe9..9b641b38b857 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2616,6 +2616,19 @@ void dm_get(struct mapped_device *md)
         BUG_ON(test_bit(DMF_FREEING, &md->flags));
 }
 
+int dm_hold(struct mapped_device *md)
+{
+        spin_lock(&_minor_lock);
+        if (test_bit(DMF_FREEING, &md->flags)) {
+                spin_unlock(&_minor_lock);
+                return -EBUSY;
+        }
+        dm_get(md);
+        spin_unlock(&_minor_lock);
+        return 0;
+}
+EXPORT_SYMBOL_GPL(dm_hold);
+
 const char *dm_device_name(struct mapped_device *md)
 {
         return md->name;
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 2646aed1d3fe..fd23978d93fe 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -375,6 +375,7 @@ int dm_create(int minor, struct mapped_device **md);
  */
 struct mapped_device *dm_get_md(dev_t dev);
 void dm_get(struct mapped_device *md);
+int dm_hold(struct mapped_device *md);
 void dm_put(struct mapped_device *md);
 
 /*