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Re: [Qemu-devel] [PATCH v2 05/10] block: add block job transactions
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v2 05/10] block: add block job transactions |
Date: |
Wed, 8 Jul 2015 14:36:26 +0100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Wed, Jul 08, 2015 at 09:59:24AM +0800, Fam Zheng wrote:
> On Tue, 07/07 13:59, Stefan Hajnoczi wrote:
> > On Tue, Jul 07, 2015 at 03:32:45PM +0800, Fam Zheng wrote:
> > > On Mon, 07/06 15:24, Stefan Hajnoczi wrote:
> > > > +/**
> > > > + * block_job_txn_add_job:
> > > > + * @txn: The transaction (may be NULL)
> > > > + * @job: Job to add to the transaction
> > > > + *
> > > > + * Add @job to the transaction. The @job must not already be in a
> > > > transaction.
> > > > + * The block job driver must call block_job_txn_prepare_to_complete()
> > > > before
> > >
> > > s/block_job_txn_prepare_to_complete/block_job_txn_job_done/
> > >
> > > Reading this for a second time I start to feel it too complicated for the
> > > good.
> > >
> > > I have another idea: in block_job_completed, check if other jobs have
> > > failed,
> > > and call this job driver's (imaginary) "abort()" callback accordingly; if
> > > all
> > > jobs has succeeded, call a "commit" callback during last
> > > block_job_completed.
> > >
> > > Does that make sense?
> >
> > I think you've skipped the hard part: immediate cancellation. If a job
> > is cancelled by the user or a job fails, then all other jobs are
> > cancelled immediately.
> >
> > Immediate cancellation has the problem that jobs could be running in any
> > AioContext, so you need to handle concurrency. That's where the
> > locking, juggling AioContexts, and interaction between blockjobs comes
> > in.
>
> OK, let me try again:
>
> The idea is intercepting job->cb so we can handle jobs completely in
> block_job_completed (which is in main loop), rather than the coroutines that
> can be any AioContext.
>
> 1) If a job is cancelled or failed, it goes to block_job_completed
> immediately,
> with block_job_is_cancelled() == true. In this case, we call
> block_job_cancel_sync on all other jobs, and then call "abort()" callbacks to
> reclaim any bitmaps, then emit QMP events. Some other jobs may have already
> completed before this point, but it's not a problem because we always defer
> the
> actual completions (abort/commit and QMP) altogether.
>
> 2) If there is no job failed or canceled, in the last block_job_completed, we
> call "commit()" to abdicate bitmaps, and emit the QMP events.
>
> This would still require BlockJobTxn to track the block jobs in a group, but
> hopefully it could reduce the complexity of interactions between block jobs.
>
> I can prototype it if this isn't missing anything obvious.
Yes, please try it. It's half-way between what John originally did and
what I did. It might be the simplest solution.
Be careful with the final piece of code used to complete jobs from
block_job_defer_to_main_loop(). It runs from a BH in the main loop
after the coroutine has terminated. In the fail/cancel case you might
need to protect against race conditions - especially if two jobs finish
in the same event loop iteration.
I didn't handle that since block_job_txn_job_done() is called while the
coroutine is still alive.
pgpSWMv0rAGMw.pgp
Description: PGP signature
- [Qemu-devel] [PATCH v2 03/10] block: rename BlkTransactionState and BdrvActionOps, (continued)
- [Qemu-devel] [PATCH v2 03/10] block: rename BlkTransactionState and BdrvActionOps, Stefan Hajnoczi, 2015/07/06
- [Qemu-devel] [PATCH v2 04/10] block: keep bitmap if incremental backup job is cancelled, Stefan Hajnoczi, 2015/07/06
- [Qemu-devel] [PATCH v2 06/10] blockdev: make BlockJobTxn available to qmp 'transaction', Stefan Hajnoczi, 2015/07/06
- [Qemu-devel] [PATCH v2 07/10] block/backup: support block job transactions, Stefan Hajnoczi, 2015/07/06
- [Qemu-devel] [PATCH v2 05/10] block: add block job transactions, Stefan Hajnoczi, 2015/07/06
[Qemu-devel] [PATCH v2 08/10] iotests: 124 - transactional failure test, Stefan Hajnoczi, 2015/07/06
[Qemu-devel] [PATCH v2 09/10] qmp-commands.hx: Update the supported 'transaction' operations, Stefan Hajnoczi, 2015/07/06
[Qemu-devel] [PATCH v2 10/10] tests: add BlockJobTxn unit test, Stefan Hajnoczi, 2015/07/06