Re: [PATCH 2/3] block: Introduce blk_rq_completed()

[Paolo Bonzini <pbonzini@redhat.com>]

Il 27/05/2014 12:59, James Bottomley ha scritto:
> On Tue, 2014-05-27 at 12:47 +0200, Paolo Bonzini wrote:
>> Il 27/05/2014 12:21, James Bottomley ha scritto:
>>> I could also see us one day extending the TMF capability to abort any
>>> running command, which would make even an assertion of block timed out
>>> or completed invalid.
>>
>> Actually the assertion would remain valid, and that's exactly what Bart
>> wants to document with this assertion.
>
> No, it wouldn't: if we abort a running command by definition the command
> hadn't timed out and might not be completed.  This is required by TMF
> handling because now you have an abort racing with a completion.  Either
> the command completes normally because it misses the abort or the abort
> gets to it and its returned status is set to TASK_ABORTED.  That's the
> only way you can tell if the abort was successful or not.
>
> If you're thinking we would tell block to ignore returning commands
> before issuing the abort, we'd never be able to tell if the abort were
> successful, so we have to allow the race to collect the status.

You could use a different mechanism than a softirq to tell the abort 
were successful, for example by overriding scsi_done.  But with respect 
to the block layer, the mechanics of avoiding the race and double-free 
would probably be the same.

Paolo