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

[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 <address@hidden>
>>> 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.

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.

Paolo