Re: [Qemu-devel] [PATCH] [RFC] aio/async: Add timed bottom-halves

[Alex Bligh <alex@alex.org.uk>]

Paolo,

--On 15 July 2013 22:53:17 +0200 Paolo Bonzini <pbonzini@redhat.com> wrote:

> So far you are right.
>
> But this only happens if qemu_aio_wait() actually returns, so that on
> the next call we poll for timers.  If QEMU is stuck in qemu_aio_wait()'s
> infinite-timeout poll(), it will never advance and process the timed
> bottom halves.

I may have misunderstood the code here. I thought what it was doing was
setting the timeout to poll() to 0 if there was a bh queued, 10ms if
there was a bh->idle timeout queued, or infinite otherwise.

What I changed it to do was set the timeout to:
* 0 if there was an untimed bh ready (no change)
* 10ms if there was an untimed idle bh ready (no change)
* the number of ms to expiry if there is a timed bh ready (rounded up)
* infinite otherwise (no change)
* and if there is more the one, the minimum of those

So the infinite timeout poll should not be entered if there is a timed
bh there. This of course assumes a single thread as else there is a TOCTOU
problem if a timed bh gets inserted between calculating the expiry time
and the poll.

> This goes to the question of having aio_notify() or not.  If you have
> it, you will immediately process timed BHs that are "born expired".  For
> other bottom halves, there will be no difference if you add it or not.

You've lost me there. I'm taking it by 'born expired' you mean
when poll() is entered, they're already expired. Timed BH's that are
'born expired' should (with the patch I sent) be treated exactly the
same as untimed BH's, i.e. the timeout to poll should be being
set to zero.

-- 
Alex Bligh