On Wed, Oct 09, 2013 at 12:16:47PM +0200, Paolo Bonzini wrote:
> Il 09/10/2013 11:55, Stefan Hajnoczi ha scritto:
> > aio_context_acquire() and aio_context_release() make it possible for
> > multiple
> > threads to safely operate on a shared AioContext. This is a prerequisite
> > for
> > using the block layer outside the QEMU global mutex.
> >
> > Imagine that a dataplane thread is performing I/O on behalf of the guest
> > when
> > the user issues a monitor command that needs to access the BlockDriverState.
> > The monitor thread needs to acquire the AioContext before calling bdrv_*()
> > functions on it. This prevents the dataplane thread from interfering with
> > the
> > monitor thread.
> >
> > There was a discussion in the RFC email thread about how to implement
> > fairness:
> > each thread should get a turn to acquire the AioContext so that starvation
> > is
> > avoided.
> >
> > Paolo suggested doing what the QEMU main loop does today: drop the lock
> > before
> > invoking ppoll(2). It turns out that this is tricky since we want both
> > threads
> > to be able to call aio_poll() simultaneously. AioContext->pollfds[]
> > currently
> > prevents two simultaneous aio_poll() calls since it is a shared resource.
> > It's
> > also tricky to avoid deadlocks if two threads execute aio_poll()
> > simulatenously
> > except by waking all threads each time *any* thread makes any progress.
> >
> > I found the simplest solution is to implement RFifoLock, a recursive lock
> > with
> > FIFO ordering. This lock supports the semantics we need for the following
> > pattern:
> >
> > /* Event loop thread */
> > while (running) {
> > aio_context_acquire(ctx);
> > aio_poll(ctx, true);
> > aio_context_release(ctx);
> > }
> >
> > /* Another thread */
> > aio_context_acquire(ctx);
> > bdrv_read(bs, 0x1000, buf, 1);
> > aio_context_release(ctx);
>
> Should bdrv_read itself call aio_context_acquire? If not, what's the
> use of recursion?
Recursion is an artifact of our previous discussions. In a few days I
should have explored enough use cases to know whether we need it or not
- I'm converting code to use acquire/release right now.
Will let you know once I've figured out if we really need it or not.
Stefan
