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Re: [Qemu-devel] backup bug or question
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] backup bug or question |
Date: |
Mon, 12 Aug 2019 15:23:52 +0200 |
User-agent: |
Mutt/1.11.3 (2019-02-01) |
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