[Qemu-devel] backup bug or question

[Vladimir Sementsov-Ogievskiy]

Hi!

Hmm, hacking around backup I have a question:

What prevents guest write request after job_start but before setting
write notifier?

code path:

qmp_drive_backup or transaction with backup

    job_start
       aio_co_enter(job_co_entry) /* may only schedule execution, isn't it ? */

....

job_co_entry
    job_pause_point() /* it definitely yields, isn't it bad? */
    job->driver->run() /* backup_run */

----

backup_run()
    bdrv_add_before_write_notifier()

...

And what guarantees we give to the user? Is it guaranteed that write notifier is
set when qmp command returns?

And I guess, if we start several backups in a transaction it should be 
guaranteed
that the set of backups is consistent and correspond to one point in time...

-- 
Best regards,
Vladimir