Re: [Qemu-devel] [PATCH v5 08/13] blockdev: Block device IO during internal snapshot transaction

[Max Reitz]

On 20.05.2015 08:16, Fam Zheng wrote:

Signed-off-by: Fam Zheng <address@hidden>
---
 blockdev.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/blockdev.c b/blockdev.c
index 5eaf77e..7f763d9 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1262,6 +1262,7 @@ typedef struct InternalSnapshotState {
     BlockDriverState *bs;
     AioContext *aio_context;
     QEMUSnapshotInfo sn;
+    Error *blocker;
 } InternalSnapshotState;
 
 static void internal_snapshot_prepare(BlkTransactionState *common,
@@ -1300,6 +1301,10 @@ static void internal_snapshot_prepare(BlkTransactionState *common,
     state->aio_context = bdrv_get_aio_context(bs);
     aio_context_acquire(state->aio_context);
 
+    state->bs = bs;
+    error_setg(&state->blocker, "internal snapshot in progress");
+    bdrv_op_block(bs, BLOCK_OP_TYPE_DEVICE_IO, state->blocker);
+
     if (!bdrv_is_inserted(bs)) {
         error_set(errp, QERR_DEVICE_HAS_NO_MEDIUM, device);
         return;
@@ -1354,9 +1359,6 @@ static void internal_snapshot_prepare(BlkTransactionState *common,
                          name, device);
         return;
     }
-
-    /* 4. succeed, mark a snapshot is created */
-    state->bs = bs;
 }

As far as I can see, the failed operation in a transaction is aborted, too. So with this pulled up, if the creation of the snapshot failed, internal_snapshot_abort() will try to delete the (non-existing) snapshot which will fail. What is saving us from an even worse fate (internal_snapshot_prepare() failing because a snapshot with the name already existed, which would then be deleted by internal_snapshot_abort()) is that sn->name will not be set until the snapshot is actually attempted to be taken (thus, bdrv_snapshot_delete() in internal_snapshot_abort() fails). Oh, and that sn->id_str will only be set by bdrv_snapshot_create() in case of success.

So the only visible result is " Failed to delete snapshot with id '' and name '' on device 'disk' in abort: Can't find the snapshot".

One way of fixing this would probably to check whether sn->id_str[0] is set instead of state->bs in internal_snapshot_abort(). That is, if bdrv_snapshot_create() really always and only fills sn->id_str if it was successful.

Max

 
 static void internal_snapshot_abort(BlkTransactionState *common)
@@ -1387,6 +1389,10 @@ static void internal_snapshot_clean(BlkTransactionState *common)
     InternalSnapshotState *state = DO_UPCAST(InternalSnapshotState,
                                              common, common);
 
+    if (state->bs) {
+        bdrv_op_unblock(state->bs, BLOCK_OP_TYPE_DEVICE_IO, state->blocker);
+        error_free(state->blocker);
+    }
     if (state->aio_context) {
         aio_context_release(state->aio_context);
     }