Re: [Qemu-devel] [PATCH v3 5/6] blockjob: refactor backup_start as backup_job_create

[John Snow <jsnow@redhat.com>]

On 11/03/2016 09:17 AM, Kevin Wolf wrote:
> Am 02.11.2016 um 18:50 hat John Snow geschrieben:
>> 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>
>
>> +static void drive_backup_commit(BlkActionState *common)
>> +{
>> +    DriveBackupState *state = DO_UPCAST(DriveBackupState, common, common);
>> +    if (state->job) {
>> +        block_job_start(state->job);
>> +    }
>>  }
>
> How could state->job ever be NULL?
>

Mechanical thinking. It can't. (I definitely didn't copy paste from the 
.abort routines. Definitely.)

> Same question for abort, and for blockdev_backup_commit/abort.
>

Abort ... we may not have created the job successfully. Abort gets 
called whether or not we made it to or through the matching .prepare.

> Kevin
>

--js