Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

On Tue, Sep 04, 2012 at 01:18:31PM +0200, Paolo Bonzini wrote:

Il 04/09/2012 13:09, Michael S. Tsirkin ha scritto:

quoted

quoted

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    queuecommand on CPU #0         queuecommand #2 on CPU #1
  --------------------------------------------------------------
    atomic_inc_return(...) == 1
                                   atomic_inc_return(...) == 2
                                   virtscsi_queuecommand to queue #1
    tgt->req_vq = queue #0
    virtscsi_queuecommand to queue #0

then two requests are issued to different queues without a quiescent
point in the middle.

What happens then? Does this break correctness?

Yes, requests to the same target should be processed in FIFO order, or
you have things like a flush issued before the write it was supposed to
flush.  This is why I can only change the queue when there is no request
pending.

Paolo

I see.  I guess you can rewrite this as:
atomic_inc
if (atomic_read() == 1)
which is a bit cheaper, and make the fact
that you do not need increment and return to be atomic,
explicit.

Another simple idea: store last processor id in target,
if it is unchanged no need to play with req_vq
and take spinlock.

Also - some kind of comment explaining why a similar race can not happen
with this lock in place would be nice: I see why this specific race can
not trigger but since lock is dropped later before you submit command, I
have hard time convincing myself what exactly gurantees that vq is never
switched before or even while command is submitted.




-- 
MST