blockjob: refactor backup_start as backup_job_create

Refactor backup_start as backup_job_create, which only creates the job,
but does not automatically start it. The old interface, 'backup_start',
is not kept in favor of limiting the number of nearly-identical interfaces
that would have to be edited to keep up with QAPI changes in the future.

Callers that wish to synchronously start the backup_block_job can
instead just call block_job_start immediately after calling
backup_job_create.

Transactions are updated to use the new interface, calling block_job_start
only during the .commit phase, which helps prevent race conditions where
jobs may finish before we even finish building the transaction. This may
happen, for instance, during empty block backup jobs.

Reported-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1478587839-9834-6-git-send-email-jsnow@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>

block/backup.c

@@ -543,7 +543,7 @@ static const BlockJobDriver backup_job_driver = {
     .drain                  = backup_drain,
 };
 
-void backup_start(const char *job_id, BlockDriverState *bs,
+BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, int64_t speed,
                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
                   bool compress,
@@ -563,52 +563,52 @@ void backup_start(const char *job_id, BlockDriverState *bs,
 
     if (bs == target) {
         error_setg(errp, "Source and target cannot be the same");
-        return;
+        return NULL;
     }
 
     if (!bdrv_is_inserted(bs)) {
         error_setg(errp, "Device is not inserted: %s",
                    bdrv_get_device_name(bs));
-        return;
+        return NULL;
     }
 
     if (!bdrv_is_inserted(target)) {
         error_setg(errp, "Device is not inserted: %s",
                    bdrv_get_device_name(target));
-        return;
+        return NULL;
     }
 
     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
         error_setg(errp, "Compression is not supported for this drive %s",
                    bdrv_get_device_name(target));
-        return;
+        return NULL;
     }
 
     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
-        return;
+        return NULL;
     }
 
     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
-        return;
+        return NULL;
     }
 
     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
         if (!sync_bitmap) {
             error_setg(errp, "must provide a valid bitmap name for "
                              "\"incremental\" sync mode");
-            return;
+            return NULL;
         }
 
         /* Create a new bitmap, and freeze/disable this one. */
         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
-            return;
+            return NULL;
         }
     } else if (sync_bitmap) {
         error_setg(errp,
                    "a sync_bitmap was provided to backup_run, "
                    "but received an incompatible sync_mode (%s)",
                    MirrorSyncMode_lookup[sync_mode]);
-        return;
+        return NULL;
     }
 
     len = bdrv_getlength(bs);
@@ -655,8 +655,8 @@ void backup_start(const char *job_id, BlockDriverState *bs,
     block_job_add_bdrv(&job->common, target);
     job->common.len = len;
     block_job_txn_add_job(txn, &job->common);
-    block_job_start(&job->common);
-    return;
+
+    return &job->common;
 
  error:
     if (sync_bitmap) {
@@ -666,4 +666,6 @@ void backup_start(const char *job_id, BlockDriverState *bs,
         backup_clean(&job->common);
         block_job_unref(&job->common);
     }
+
+    return NULL;
 }

block/replication.c

@@ -421,6 +421,7 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
     int64_t active_length, hidden_length, disk_length;
     AioContext *aio_context;
     Error *local_err = NULL;
+    BlockJob *job;
 
     aio_context = bdrv_get_aio_context(bs);
     aio_context_acquire(aio_context);
@@ -508,17 +509,18 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
         bdrv_op_block_all(top_bs, s->blocker);
         bdrv_op_unblock(top_bs, BLOCK_OP_TYPE_DATAPLANE, s->blocker);
 
-        backup_start(NULL, s->secondary_disk->bs, s->hidden_disk->bs, 0,
-                     MIRROR_SYNC_MODE_NONE, NULL, false,
-                     BLOCKDEV_ON_ERROR_REPORT, BLOCKDEV_ON_ERROR_REPORT,
-                     BLOCK_JOB_INTERNAL, backup_job_completed, bs,
-                     NULL, &local_err);
+        job = backup_job_create(NULL, s->secondary_disk->bs, s->hidden_disk->bs,
+                                0, MIRROR_SYNC_MODE_NONE, NULL, false,
+                                BLOCKDEV_ON_ERROR_REPORT,
+                                BLOCKDEV_ON_ERROR_REPORT, BLOCK_JOB_INTERNAL,
+                                backup_job_completed, bs, NULL, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
             backup_job_cleanup(bs);
             aio_context_release(aio_context);
             return;
         }
+        block_job_start(job);
         break;
     default:
         aio_context_release(aio_context);

blockdev.c

@@ -1811,7 +1811,7 @@ typedef struct DriveBackupState {
     BlockJob *job;
 } DriveBackupState;
 
-static void do_drive_backup(DriveBackup *backup, BlockJobTxn *txn,
+static BlockJob *do_drive_backup(DriveBackup *backup, BlockJobTxn *txn,
                             Error **errp);
 
 static void drive_backup_prepare(BlkActionState *common, Error **errp)
@@ -1835,23 +1835,26 @@ static void drive_backup_prepare(BlkActionState *common, Error **errp)
     bdrv_drained_begin(bs);
     state->bs = bs;
 
-    do_drive_backup(backup, common->block_job_txn, &local_err);
+    state->job = do_drive_backup(backup, common->block_job_txn, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         return;
     }
+}
 
-    state->job = state->bs->job;
+static void drive_backup_commit(BlkActionState *common)
+{
+    DriveBackupState *state = DO_UPCAST(DriveBackupState, common, common);
+    assert(state->job);
+    block_job_start(state->job);
 }
 
 static void drive_backup_abort(BlkActionState *common)
 {
     DriveBackupState *state = DO_UPCAST(DriveBackupState, common, common);
-    BlockDriverState *bs = state->bs;
 
-    /* Only cancel if it's the job we started */
-    if (bs && bs->job && bs->job == state->job) {
-        block_job_cancel_sync(bs->job);
+    if (state->job) {
+        block_job_cancel_sync(state->job);
     }
 }
 
@@ -1872,8 +1875,8 @@ typedef struct BlockdevBackupState {
     AioContext *aio_context;
 } BlockdevBackupState;
 
-static void do_blockdev_backup(BlockdevBackup *backup, BlockJobTxn *txn,
-                               Error **errp);
+static BlockJob *do_blockdev_backup(BlockdevBackup *backup, BlockJobTxn *txn,
+                                    Error **errp);
 
 static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
 {
@@ -1906,23 +1909,26 @@ static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
     state->bs = bs;
     bdrv_drained_begin(state->bs);
 
-    do_blockdev_backup(backup, common->block_job_txn, &local_err);
+    state->job = do_blockdev_backup(backup, common->block_job_txn, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         return;
     }
+}
 
-    state->job = state->bs->job;
+static void blockdev_backup_commit(BlkActionState *common)
+{
+    BlockdevBackupState *state = DO_UPCAST(BlockdevBackupState, common, common);
+    assert(state->job);
+    block_job_start(state->job);
 }
 
 static void blockdev_backup_abort(BlkActionState *common)
 {
     BlockdevBackupState *state = DO_UPCAST(BlockdevBackupState, common, common);
-    BlockDriverState *bs = state->bs;
 
-    /* Only cancel if it's the job we started */
-    if (bs && bs->job && bs->job == state->job) {
-        block_job_cancel_sync(bs->job);
+    if (state->job) {
+        block_job_cancel_sync(state->job);
     }
 }
 
@@ -2072,12 +2078,14 @@ static const BlkActionOps actions[] = {
     [TRANSACTION_ACTION_KIND_DRIVE_BACKUP] = {
         .instance_size = sizeof(DriveBackupState),
         .prepare = drive_backup_prepare,
+        .commit = drive_backup_commit,
         .abort = drive_backup_abort,
         .clean = drive_backup_clean,
     },
     [TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP] = {
         .instance_size = sizeof(BlockdevBackupState),
         .prepare = blockdev_backup_prepare,
+        .commit = blockdev_backup_commit,
         .abort = blockdev_backup_abort,
         .clean = blockdev_backup_clean,
     },
@@ -3106,11 +3114,13 @@ out:
     aio_context_release(aio_context);
 }
 
-static void do_drive_backup(DriveBackup *backup, BlockJobTxn *txn, Error **errp)
+static BlockJob *do_drive_backup(DriveBackup *backup, BlockJobTxn *txn,
+                                 Error **errp)
 {
     BlockDriverState *bs;
     BlockDriverState *target_bs;
     BlockDriverState *source = NULL;
+    BlockJob *job = NULL;
     BdrvDirtyBitmap *bmap = NULL;
     AioContext *aio_context;
     QDict *options = NULL;
@@ -3139,7 +3149,7 @@ static void do_drive_backup(DriveBackup *backup, BlockJobTxn *txn, Error **errp)
 
     bs = qmp_get_root_bs(backup->device, errp);
     if (!bs) {
-        return;
+        return NULL;
     }
 
     aio_context = bdrv_get_aio_context(bs);
@@ -3213,10 +3223,10 @@ static void do_drive_backup(DriveBackup *backup, BlockJobTxn *txn, Error **errp)
         }
     }
 
-    backup_start(backup->job_id, bs, target_bs, backup->speed, backup->sync,
-                 bmap, backup->compress, backup->on_source_error,
-                 backup->on_target_error, BLOCK_JOB_DEFAULT,
-                 NULL, NULL, txn, &local_err);
+    job = backup_job_create(backup->job_id, bs, target_bs, backup->speed,
+                            backup->sync, bmap, backup->compress,
+                            backup->on_source_error, backup->on_target_error,
+                            BLOCK_JOB_DEFAULT, NULL, NULL, txn, &local_err);
     bdrv_unref(target_bs);
     if (local_err != NULL) {
         error_propagate(errp, local_err);
@@ -3225,11 +3235,17 @@ static void do_drive_backup(DriveBackup *backup, BlockJobTxn *txn, Error **errp)
 
 out:
     aio_context_release(aio_context);
+    return job;
 }
 
 void qmp_drive_backup(DriveBackup *arg, Error **errp)
 {
-    return do_drive_backup(arg, NULL, errp);
+
+    BlockJob *job;
+    job = do_drive_backup(arg, NULL, errp);
+    if (job) {
+        block_job_start(job);
+    }
 }
 
 BlockDeviceInfoList *qmp_query_named_block_nodes(Error **errp)
@@ -3237,12 +3253,14 @@ BlockDeviceInfoList *qmp_query_named_block_nodes(Error **errp)
     return bdrv_named_nodes_list(errp);
 }
 
-void do_blockdev_backup(BlockdevBackup *backup, BlockJobTxn *txn, Error **errp)
+BlockJob *do_blockdev_backup(BlockdevBackup *backup, BlockJobTxn *txn,
+                             Error **errp)
 {
     BlockDriverState *bs;
     BlockDriverState *target_bs;
     Error *local_err = NULL;
     AioContext *aio_context;
+    BlockJob *job = NULL;
 
     if (!backup->has_speed) {
         backup->speed = 0;
@@ -3262,7 +3280,7 @@ void do_blockdev_backup(BlockdevBackup *backup, BlockJobTxn *txn, Error **errp)
 
     bs = qmp_get_root_bs(backup->device, errp);
     if (!bs) {
-        return;
+        return NULL;
     }
 
     aio_context = bdrv_get_aio_context(bs);
@@ -3284,20 +3302,25 @@ void do_blockdev_backup(BlockdevBackup *backup, BlockJobTxn *txn, Error **errp)
             goto out;
         }
     }
-    backup_start(backup->job_id, bs, target_bs, backup->speed, backup->sync,
-                 NULL, backup->compress, backup->on_source_error,
-                 backup->on_target_error, BLOCK_JOB_DEFAULT,
-                 NULL, NULL, txn, &local_err);
+    job = backup_job_create(backup->job_id, bs, target_bs, backup->speed,
+                            backup->sync, NULL, backup->compress,
+                            backup->on_source_error, backup->on_target_error,
+                            BLOCK_JOB_DEFAULT, NULL, NULL, txn, &local_err);
     if (local_err != NULL) {
         error_propagate(errp, local_err);
     }
 out:
     aio_context_release(aio_context);
+    return job;
 }
 
 void qmp_blockdev_backup(BlockdevBackup *arg, Error **errp)
 {
-    do_blockdev_backup(arg, NULL, errp);
+    BlockJob *job;
+    job = do_blockdev_backup(arg, NULL, errp);
+    if (job) {
+        block_job_start(job);
+    }
 }
 
 /* Parameter check and block job starting for drive mirroring.

include/block/block_int.h

@@ -748,7 +748,7 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
                   bool unmap, Error **errp);
 
 /*
- * backup_start:
+ * backup_job_create:
  * @job_id: The id of the newly-created job, or %NULL to use the
  * device name of @bs.
  * @bs: Block device to operate on.
@@ -764,18 +764,19 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
  * @opaque: Opaque pointer value passed to @cb.
  * @txn: Transaction that this job is part of (may be NULL).
  *
- * Start a backup operation on @bs.  Clusters in @bs are written to @target
+ * Create a backup operation on @bs.  Clusters in @bs are written to @target
  * until the job is cancelled or manually completed.
  */
-void backup_start(const char *job_id, BlockDriverState *bs,
-                  BlockDriverState *target, int64_t speed,
-                  MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
-                  bool compress,
-                  BlockdevOnError on_source_error,
-                  BlockdevOnError on_target_error,
-                  int creation_flags,
-                  BlockCompletionFunc *cb, void *opaque,
-                  BlockJobTxn *txn, Error **errp);
+BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
+                            BlockDriverState *target, int64_t speed,
+                            MirrorSyncMode sync_mode,
+                            BdrvDirtyBitmap *sync_bitmap,
+                            bool compress,
+                            BlockdevOnError on_source_error,
+                            BlockdevOnError on_target_error,
+                            int creation_flags,
+                            BlockCompletionFunc *cb, void *opaque,
+                            BlockJobTxn *txn, Error **errp);
 
 void hmp_drive_add_node(Monitor *mon, const char *optstr);