Re: [Qemu-devel] [PATCH v11 01/19] block: Add bdrv_drain_all_{begin, end}()

[Paolo Bonzini]

On 25/10/2016 15:39, Alberto Garcia wrote:
> On Mon 24 Oct 2016 12:53:41 PM CEST, Paolo Bonzini wrote:
> 
>>> My first thoughts were about how to let an unpause succeed without a
>>> previous pause for these objects, but actually I think this isn't
>>> what we should do. We rather want to actually do the pause instead
>>> because even new BDSes and block jobs should probably start in a
>>> quiesced state when created inside a drain_all section.
>>
>> Yes, I agree with this.  It shouldn't be too hard to implement it.  It
>> would require a BlockDriverState to look at the global "inside
>> bdrv_drain_all_begin" state, and ask its BlockBackend to disable
>> itself upon bdrv_replace_child.
> 
> Why in bdrv_replace_child()? bdrv_drain_all_end() enables all BDSs, but
> if you add one with "blockdev-add" it's not going to be disabled using
> this method.

You only need to disable it when blk_insert_bs is called.  In fact...

> In addition to that block jobs need the same, don't they? Something like
> "job->pause_count = all_quiesce_counter" in the initialization.

... we discussed a couple weeks ago that block jobs should register
BlkDeviceOps that pause the job in the drained_begin callback.  So when
the block job calls blk_insert_bs, the drained_begin callback would
start the job as paused.

> I think we'd also need to add block_job_pause_point() at the beginning
> of each one of their coroutines, in order to make sure that they really
> start paused.

It depends on the job, for example streaming starts with
block_job_sleep_ns.  Commit also does, except for some blk_getlength and
blk_truncate calls that could be moved to the caller.

Paolo