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

[John Snow]

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 <address@hidden>
> > Signed-off-by: John Snow <address@hidden>
>
> > +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