Re: [Qemu-devel] [RFC] dataplane: IOThreads and writing dataplane-capable code

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Stefan Hajnoczi (stefanha@redhat.com) wrote:

<snip>

> How to synchronize with an IOThread
> -----------------------------------
> AioContext is not thread-safe so some rules must be followed when using file
> descriptors, event notifiers, timers, or BHs across threads:
> 
> 1. AioContext functions can be called safely from file descriptor, event
> notifier, timer, or BH callbacks invoked by the AioContext.  No locking is
> necessary.
> 
> 2. Other threads wishing to access the AioContext must use
> aio_context_acquire()/aio_context_release() for mutual exclusion.  Once the
> context is acquired no other thread can access it or run event loop iterations
> in this AioContext.
> 
> aio_context_acquire()/aio_context_release() calls may be nested.  This
> means you can call them if you're not sure whether #1 applies.
> 
> Side note: the best way to schedule a function call across threads is to create
> a BH in the target AioContext beforehand and then call qemu_bh_schedule().  No
> acquire/release or locking is needed for the qemu_bh_schedule() call.  But be
> sure to acquire the AioContext for aio_bh_new() if necessary.

How do these IOThreads pause during migration?
Are they paused by the 'qemu_mutex_lock_iothread' that the migration thread calls?

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK