On Thu, May 8, 2014 at 3:44 PM, Dr. David Alan Gilbert <dgilb...@redhat.com> wrote: > * Stefan Hajnoczi (stefa...@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?
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
