From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49953) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YxXnp-0002Oh-JS for qemu-devel@nongnu.org; Wed, 27 May 2015 05:39:10 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YxXno-0001qt-Be for qemu-devel@nongnu.org; Wed, 27 May 2015 05:39:09 -0400 Message-ID: <556590B1.2040405@redhat.com> Date: Wed, 27 May 2015 11:38:57 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1432711146-28405-1-git-send-email-famz@redhat.com> In-Reply-To: <1432711146-28405-1-git-send-email-famz@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC PATCH 0/4] aio: Don't poll ioeventfd in nested aio_poll() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Fam Zheng , qemu-devel@nongnu.org Cc: Kevin Wolf , Stefan Weil , qemu-block@nongnu.org, Stefan Hajnoczi , "Michael S. Tsirkin" On 27/05/2015 09:19, Fam Zheng wrote: > This series looks at the other side of the broken "qmp transaction" problem > with dataplane [1] - the event loop. > > Before, an ioeventfd of a non-dataplane device is registered via > qemu_set_fd_handler, which is only polled directly by main_loop_wait(), not in > aio_poll(); an ioeventfd of a dataplane device (virtio-blk/virtio-scsi) is > registered specially via aio_set_event_notifier, which is a wrapper of > aio_set_fd_handler, thus it WILL be polled by all aio_poll(). > > As explained in [1], and in patch 3, this is wrong. The handlers mustn't run. > > [1] Fixes it by unregistering them temporarily around such nested poll, while > this series fixes it by skipping those file descriptors in all nested > aio_poll(), just like how iohandler behaves in the main loop. > > On the one hand, it is simpler than [1]; on the other hand, this approach is > also interesting because once we remove qemu_set_fd_handler2 [2], iohandler.c > can be removed by converting all qemu_set_fd_handler to the new > aio_set_io_event_notifier introduced in this series. I don't think this can work. Whenever the main context is doing synchronous work for dataplane, it is in the outermost aio_poll. Paolo