Re: [Qemu-devel] [RFC] aio: add aio_context_acquire() and aio_context_release()

[Paolo Bonzini]

Il 27/08/2013 16:39, Stefan Hajnoczi ha scritto:
> +void aio_context_acquire(AioContext *ctx)
> +{
> +    qemu_mutex_lock(&ctx->acquire_lock);
> +    while (ctx->owner) {
> +        assert(!qemu_thread_is_self(ctx->owner));
> +        aio_notify(ctx); /* kick current owner */
> +        qemu_cond_wait(&ctx->acquire_cond, &ctx->acquire_lock);
> +    }
> +    qemu_thread_get_self(&ctx->owner_thread);
> +    ctx->owner = &ctx->owner_thread;
> +    qemu_mutex_unlock(&ctx->acquire_lock);
> +}
> +
> +void aio_context_release(AioContext *ctx)
> +{
> +    qemu_mutex_lock(&ctx->acquire_lock);
> +    assert(ctx->owner && qemu_thread_is_self(ctx->owner));
> +    ctx->owner = NULL;
> +    qemu_cond_signal(&ctx->acquire_cond);
> +    qemu_mutex_unlock(&ctx->acquire_lock);
> +}

Thinking more about it, there is a risk of busy waiting here if one
thread releases the AioContext and tries to acquire it again (as in the
common case of one thread doing acquire/poll/release in a loop).  It
would only work if mutexes guarantee some level of fairness.

If you implement recursive acquisition, however, you can make aio_poll
acquire the context up until just before it invokes ppoll, and then
again after it comes back from the ppoll.  The two acquire/release pair
will be no-ops if called during "synchronous" I/O such as

  /* Another thread */
  aio_context_acquire(ctx);
  bdrv_read(bs, 0x1000, buf, 1);
  aio_context_release(ctx);

Yet they will do the right thing when called from the event loop thread.

(where the bdrv_read can actually be something more complicated such as
a live snapshot or, in general, anything involving bdrv_drain_all).

Paolo