Re: [Qemu-devel] [PATCH v2 0/2] block: make aio_poll(ctx, true) block with no fds

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Nov 26, 2013 at 04:17:59PM +0100, Stefan Hajnoczi wrote:
> v2:
>  * Oops, this patch got lost.  Rebased onto qemu.git/master for 1.8
> 
> Jan and Alex have expressed that aio_poll(ctx, blocking=true) should block even
> when there are no file descriptors registered.  This can be handy since other
> threads may still kick the AioContext using aio_notify(ctx).
> 
> A concrete example is a thread that has only a timer in its AioContext.
> aio_poll(ctx, true) should block until the timer expires or another thread
> invokes aio_notify(ctx).
> 
> Alex and Paolo were concerned about bdrv_drain_all() which has the following
> comment:
> 
>   while (busy) {
>       /* FIXME: We do not have timer support here, so this is effectively
>        * a busy wait.
>        */
>       QTAILQ_FOREACH(bs, &bdrv_states, list) {
>           if (bdrv_start_throttled_reqs(bs)) {
>               busy = true;
>           }
>       }
> 
>       busy = bdrv_requests_pending_all();
>       busy |= aio_poll(qemu_get_aio_context(), busy);
>   }
> 
> Patch 1 drops this outdated comment.  The new I/O throttling code already
> eliminated the busy wait.
> 
> Patch 2 drops the special case which returns immediately from aio_poll(ctx,
> true) when no file descriptors are registered.
> 
> Note that aio_notify(ctx) still causes aio_poll(ctx, true) to return false.  I
> don't see a need to change it so aio_poll(ctx, true) always returns true.
> 
> Stefan Hajnoczi (2):
>   block: clean up bdrv_drain_all() throttling comments
>   aio: make aio_poll(ctx, true) block with no fds
> 
>  aio-posix.c      | 5 -----
>  aio-win32.c      | 5 -----
>  block.c          | 7 +------
>  tests/test-aio.c | 1 -
>  4 files changed, 1 insertion(+), 17 deletions(-)

Applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan