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

[Stefan Hajnoczi]

On Thu, May 8, 2014 at 3:44 PM, Dr. David Alan Gilbert
<address@hidden> wrote:
> * Stefan Hajnoczi (address@hidden) 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?

Currently the only IOThread user is virtio-blk data-plane.  It has a
VM state change listener registered that will stop using the IOThread
during migration.

In the future we'll have to do more than that:
It is possible to suspend all IOThreads simply by looping over
IOThread objects and calling aio_context_acquire() on their
AioContext.  You can release the AioContexts when you are done.  This
would be suitable for a "stop the world" operation for migration
hand-over.

For smaller one-off operations like block-migration.c it may also make
sense to acquire/release the AioContext.  But that's not necessary
today since dataplane is disabled during migration.

Stefan