Re: [Qemu-devel] [RFC PATCH] fix select(2) race between main_loop_wait and qemu_aio_wait

[Paolo Bonzini <pbonzini@redhat.com>]

Il 05/03/2012 10:07, Jan Kiszka ha scritto:
> > This is quite ugly.  Two threads, one running main_loop_wait and
> > one running qemu_aio_wait, can race with each other on running the
> > same iohandler.  The result is that an iohandler could run while the
> > underlying socket is not readable or writable, with possibly ill effects.
> 
> Hmm, isn't it a problem already that a socket is polled by two threads
> at the same time? Can't that be avoided?

We still have synchronous I/O in the device models.  That's the root
cause of the bug, I suppose.

> Long-term, I'd like to cut out certain file descriptors from the main
> loop and process them completely in separate threads (for separate
> locking, prioritization etc.). Dunno how NBD works, but maybe it should
> be reworked like this already.

Me too, I even made a very simple proof of concept a couple of weeks ago
(search for a thread "switching the block layer from coroutines to
threads").  It worked, though it is obviously not upstreamable in any way.

In that world order EventNotifiers would replace
qemu_aio_set_fd_handler, and socket-based protocols such as NBD would
run with blocking I/O in their own thread.  In addition to one thread
per I/O request (from a thread pool), there would be one arbiter thread
that reads replies and dispatches them to the appropriate I/O request
thread.  The arbiter thread replaces the read callback in
qemu_aio_set_fd_handler.

The problem is, even though it worked, making this thread-safe is
another story.  I suspect that in practice it is very difficult to do
without resurrecting RCU patches.

Paolo