Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Apr 01, 2022 at 01:01:53PM +0200, Paolo Bonzini wrote:
> On 4/1/22 10:05, Emanuele Giuseppe Esposito wrote:
> > > The list itself would be used internally to implement the write-side
> > > lock and unlock primitives, but it would not be protected by the above
> > > functions.  So there would be a couple additional functions:
> > > 
> > >    bdrv_graph_list_lock <-> cpu_list_lock
> > >    bdrv_graph_list_unlock <-> cpu_list_unlock
> > 
> > The list would be graph_bdrv_states, why do we need to protect it with a
> > lock? Currently it is protected by BQL, and theoretically only
> > bdrv_graph_wrlock iterates on it. And as we defined in the assertion
> > below, wrlock is always in the main loop too.
> 
> You're right, CPU_FOREACH only appears in start_exclusive; so likewise you
> only need to walk the list in bdrv_graph_wrlock, i.e. only under BQL.
> 
> My thought was that, within the implementation, you'll need a mutex to
> protect has_waiter, and protecting the list with the same mutex made sense
> to me.  But indeed it's not necessary.

What is the relationship between this new API and aio_set_fd_handler()'s
is_external?

A few thoughts:

- The new API doesn't stop more I/O requests from being submitted, it
  just blocks the current coroutine so request processing is deferred.

- In other words, is_external is a flow control API whereas the new API
  queues up request coroutines without notifying the caller.

- The new API still needs to be combined with bdrv_drained_begin/end()
  to ensure in-flight requests are done.

- It's not obvious to me whether the new API obsoletes is_external. I
  think it probably doesn't.

Stefan

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