blockjob: add .clean property

Cleaning up after we have deferred to the main thread but before the
transaction has converged can be dangerous and result in deadlocks
if the job cleanup invokes any BH polling loops.

A job may attempt to begin cleaning up, but may induce another job to
enter its cleanup routine. The second job, part of our same transaction,
will block waiting for the first job to finish, so neither job may now
make progress.

To rectify this, allow jobs to register a cleanup operation that will
always run regardless of if the job was in a transaction or not, and
if the transaction job group completed successfully or not.

Move sensitive cleanup to this callback instead which is guaranteed to
be run only after the transaction has converged, which removes sensitive
timing constraints from said cleanup.

Furthermore, in future patches these cleanup operations will be performed
regardless of whether or not we actually started the job. Therefore,
cleanup callbacks should essentially confine themselves to undoing create
operations, e.g. setup actions taken in what is now backup_start.

Reported-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1478587839-9834-3-git-send-email-jsnow@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>

1 files changed, 10 insertions, 5 deletions

diff --git a/block/backup.c b/block/backup.c
index 7b5d8a3..734a24c 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -242,6 +242,14 @@ static void backup_abort(BlockJob *job)
     }
 }
 
+static void backup_clean(BlockJob *job)
+{
+    BackupBlockJob *s = container_of(job, BackupBlockJob, common);
+    assert(s->target);
+    blk_unref(s->target);
+    s->target = NULL;
+}
+
 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
@@ -321,6 +329,7 @@ static const BlockJobDriver backup_job_driver = {
     .set_speed              = backup_set_speed,
     .commit                 = backup_commit,
     .abort                  = backup_abort,
+    .clean                  = backup_clean,
     .attached_aio_context   = backup_attached_aio_context,
     .drain                  = backup_drain,
 };
@@ -343,12 +352,8 @@ typedef struct {
 
 static void backup_complete(BlockJob *job, void *opaque)
 {
-    BackupBlockJob *s = container_of(job, BackupBlockJob, common);
     BackupCompleteData *data = opaque;
 
-    blk_unref(s->target);
-    s->target = NULL;
-
     block_job_completed(job, data->ret);
     g_free(data);
 }
@@ -658,7 +663,7 @@ void backup_start(const char *job_id, BlockDriverState *bs,
         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
     }
     if (job) {
-        blk_unref(job->target);
+        backup_clean(&job->common);
         block_job_unref(&job->common);
     }
 }