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

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v3 5/6] blockjob: refactor backup_start as backup_job_create
Date: Tue, 8 Nov 2016 10:11:19 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 08.11.2016 um 06:41 hat John Snow geschrieben:
> 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.

Ah, yes, I always forget about this. It's so counterintuitive (and
bdrv_reopen() actually works differently, it only aborts entries that
have successfully been prepared).

Is there a good reason why qmp_transaction() works this way, especially
since we have a separate .clean function?


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