Re: [Qemu-devel] [PATCH v2 06/45] block: add support for job pause/resume

[Kevin Wolf <kwolf@redhat.com>]

Am 27.09.2012 14:57, schrieb Paolo Bonzini:
> Il 27/09/2012 14:45, Kevin Wolf ha scritto:
>> Am 27.09.2012 14:27, schrieb Paolo Bonzini:
>>> Il 27/09/2012 14:18, Kevin Wolf ha scritto:
>>>>>>
>>>>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>>> I think there's a problem with terminology at least. What does "paused"
>>>> really mean? Is it that the job has been requested to pause, or that it
>>>> has actually yielded and is inactive?
>>>>
>>>> The commit message seems to use the latter semantics (which I would
>>>> consider the intuitive one),
>>>
>>> You mean this: "Paused jobs cannot be canceled without first resuming
>>> them".  I can add a specification, like "(even if the job actually has
>>> not reached the sleeping point and thus is still running)".
>>
>> I actually meant "pause happens at the next sleeping point", which isn't
>> unspecific at all.
> 
> Hmm, there are two aspects: 1) when things stop running; 2) when the job
> reports itself to be paused.  The commit message describes (1)
> precisely, and doesn't say anything about (2).  That's too specific for
> a commit message, but the header file describes it precisely.

Yes, I understood that, I just found it confusing that both were called
"paused" in different contexts.

> However, in the QMP documentation, the good comment for "bool paused;"
> must be replicated in BlockJobInfo's "paused" member.
> 
>>> From the QMP client's point of view it doesn't really matter, does it?
>>>
>>> - even after a job that writes to disk X has "really" paused, you cannot
>>> read or write disk X.  It's still owned by QEMU, it hasn't been flushed,
>>> it may play games like lazy refcounts.
>>
>> I'm not sure about this one. Consider things like a built-in NBD server.
>> Probably we'll find more cases in the future, where some monitor command
>> might seem to be safe while a job is paused.
> 
> Ok, that's a good point.  I'll add a "busy" member to BlockJobInfo.

Ok, thanks. Together with the comment from the bool paused field this
should make pretty clear what clients would have to check for.

Kevin