Re: [Qemu-block] backup bug or question

[Kevin Wolf]

Am 09.08.2019 um 15:18 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 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...

Do the patches to switch backup to a filter node solve this
automatically because that node would be inserted in
backup_job_create()?

Kevin