From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46617) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ewHRs-0000DR-6y for qemu-devel@nongnu.org; Wed, 14 Mar 2018 21:12:53 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ewHRo-00012B-Sz for qemu-devel@nongnu.org; Wed, 14 Mar 2018 21:12:52 -0400 Received: from mga18.intel.com ([134.134.136.126]:33503) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ewHRo-000118-Hr for qemu-devel@nongnu.org; Wed, 14 Mar 2018 21:12:48 -0400 Message-ID: <5AA9C944.2010403@intel.com> Date: Thu, 15 Mar 2018 09:15:48 +0800 From: Wei Wang MIME-Version: 1.0 References: <1520426065-40265-1-git-send-email-wei.w.wang@intel.com> <1520426065-40265-4-git-send-email-wei.w.wang@intel.com> <20180313183558-mutt-send-email-mst@kernel.org> <5AA88C35.90300@intel.com> <20180314045130-mutt-send-email-mst@kernel.org> <5AA8BB27.6030205@intel.com> <20180314160832-mutt-send-email-mst@kernel.org> In-Reply-To: <20180314160832-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [virtio-dev] Re: [PATCH v4 3/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, virtio-dev@lists.oasis-open.org, quintela@redhat.com, dgilbert@redhat.com, pbonzini@redhat.com, liliang.opensource@gmail.com, yang.zhang.wz@gmail.com, quan.xu0@gmail.com, nilal@redhat.com, riel@redhat.com On 03/14/2018 10:12 PM, Michael S. Tsirkin wrote: > On Wed, Mar 14, 2018 at 02:03:19PM +0800, Wei Wang wrote: >> On 03/14/2018 10:53 AM, Michael S. Tsirkin wrote: >>> On Wed, Mar 14, 2018 at 10:43:01AM +0800, Wei Wang wrote: >>>> On 03/14/2018 12:49 AM, Michael S. Tsirkin wrote: >>>>> On Wed, Mar 07, 2018 at 08:34:24PM +0800, Wei Wang wrote: >>>>> >>>>>> Signed-off-by: Wei Wang >>>>>> Signed-off-by: Liang Li >>>>>> CC: Michael S. Tsirkin >>>>>> CC: Dr. David Alan Gilbert >>>>>> CC: Juan Quintela >>>>> I find it suspicious that neither unrealize nor reset >>>>> functions have been touched at all. >>>>> Are you sure you have thought through scenarious like >>>>> hot-unplug or disabling the device by guest? >>>> OK. I think we can call balloon_free_page_stop in unrealize and reset. >>>> >>>> >>>>> +static void *virtio_balloon_poll_free_page_hints(void *opaque) >>>>> +{ >>>>> + VirtQueueElement *elem; >>>>> + VirtIOBalloon *dev = opaque; >>>>> + VirtQueue *vq = dev->free_page_vq; >>>>> + uint32_t id; >>>>> + size_t size; >>>>> What makes it safe to poke at this device from multiple threads? >>>>> I think that it would be safer to do it from e.g. BH. >>>>> >>>> Actually the free_page_optimization thread is the only user of free_page_vq, >>>> and there is only one optimization thread each time. Would this be safe >>>> enough? >>>> >>>> Best, >>>> Wei >>> Aren't there other fields there? Also things like reset affect all VQs. >>> >> Yes. But I think BHs are used to avoid re-entrancy, which isn't the issue >> here. > Since you are adding locks to address the issue - doesn't this imply > reentrancy is exactly the issue? Not really. The lock isn't intended for any reentrancy issues, since there will be only one run of the virtio_balloon_poll_free_page_hints function at any given time. Instead, the lock is used to synchronize virtio_balloon_poll_free_page_hints and virtio_balloon_free_page_stop to access dev->free_page_report_status. Please see the whole picture below: virtio_balloon_poll_free_page_hints() { while (1) { qemu_spin_lock(); if (dev->free_page_report_status >= FREE_PAGE_REPORT_S_STOP || !runstate_is_running()) { qemu_spin_unlock(); break; } ... if (id == dev->free_page_report_cmd_id) { ==> dev->free_page_report_status = FREE_PAGE_REPORT_S_START; ... qemu_spin_unlock(); } } static void virtio_balloon_free_page_stop(void *opaque) { VirtIOBalloon *s = opaque; VirtIODevice *vdev = VIRTIO_DEVICE(s); qemu_spin_lock(); ... ==> s->free_page_report_status = FREE_PAGE_REPORT_S_STOP; ... qemu_spin_unlock(); } Without the lock, there are theoretical possibilities that assigning STOP below is overridden by START above. In that case,virtio_balloon_free_page_stop does not effectively stop virtio_balloon_poll_free_page_hints. I think this issue couldn't be solved by BHs. Best, Wei From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-dev-return-3565-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [66.179.20.138]) by lists.oasis-open.org (Postfix) with ESMTP id EDCFF5818EFD for ; Wed, 14 Mar 2018 18:12:58 -0700 (PDT) Message-ID: <5AA9C944.2010403@intel.com> Date: Thu, 15 Mar 2018 09:15:48 +0800 From: Wei Wang MIME-Version: 1.0 References: <1520426065-40265-1-git-send-email-wei.w.wang@intel.com> <1520426065-40265-4-git-send-email-wei.w.wang@intel.com> <20180313183558-mutt-send-email-mst@kernel.org> <5AA88C35.90300@intel.com> <20180314045130-mutt-send-email-mst@kernel.org> <5AA8BB27.6030205@intel.com> <20180314160832-mutt-send-email-mst@kernel.org> In-Reply-To: <20180314160832-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [virtio-dev] Re: [PATCH v4 3/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT To: "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, virtio-dev@lists.oasis-open.org, quintela@redhat.com, dgilbert@redhat.com, pbonzini@redhat.com, liliang.opensource@gmail.com, yang.zhang.wz@gmail.com, quan.xu0@gmail.com, nilal@redhat.com, riel@redhat.com List-ID: On 03/14/2018 10:12 PM, Michael S. Tsirkin wrote: > On Wed, Mar 14, 2018 at 02:03:19PM +0800, Wei Wang wrote: >> On 03/14/2018 10:53 AM, Michael S. Tsirkin wrote: >>> On Wed, Mar 14, 2018 at 10:43:01AM +0800, Wei Wang wrote: >>>> On 03/14/2018 12:49 AM, Michael S. Tsirkin wrote: >>>>> On Wed, Mar 07, 2018 at 08:34:24PM +0800, Wei Wang wrote: >>>>> >>>>>> Signed-off-by: Wei Wang >>>>>> Signed-off-by: Liang Li >>>>>> CC: Michael S. Tsirkin >>>>>> CC: Dr. David Alan Gilbert >>>>>> CC: Juan Quintela >>>>> I find it suspicious that neither unrealize nor reset >>>>> functions have been touched at all. >>>>> Are you sure you have thought through scenarious like >>>>> hot-unplug or disabling the device by guest? >>>> OK. I think we can call balloon_free_page_stop in unrealize and reset. >>>> >>>> >>>>> +static void *virtio_balloon_poll_free_page_hints(void *opaque) >>>>> +{ >>>>> + VirtQueueElement *elem; >>>>> + VirtIOBalloon *dev = opaque; >>>>> + VirtQueue *vq = dev->free_page_vq; >>>>> + uint32_t id; >>>>> + size_t size; >>>>> What makes it safe to poke at this device from multiple threads? >>>>> I think that it would be safer to do it from e.g. BH. >>>>> >>>> Actually the free_page_optimization thread is the only user of free_page_vq, >>>> and there is only one optimization thread each time. Would this be safe >>>> enough? >>>> >>>> Best, >>>> Wei >>> Aren't there other fields there? Also things like reset affect all VQs. >>> >> Yes. But I think BHs are used to avoid re-entrancy, which isn't the issue >> here. > Since you are adding locks to address the issue - doesn't this imply > reentrancy is exactly the issue? Not really. The lock isn't intended for any reentrancy issues, since there will be only one run of the virtio_balloon_poll_free_page_hints function at any given time. Instead, the lock is used to synchronize virtio_balloon_poll_free_page_hints and virtio_balloon_free_page_stop to access dev->free_page_report_status. Please see the whole picture below: virtio_balloon_poll_free_page_hints() { while (1) { qemu_spin_lock(); if (dev->free_page_report_status >= FREE_PAGE_REPORT_S_STOP || !runstate_is_running()) { qemu_spin_unlock(); break; } ... if (id == dev->free_page_report_cmd_id) { ==> dev->free_page_report_status = FREE_PAGE_REPORT_S_START; ... qemu_spin_unlock(); } } static void virtio_balloon_free_page_stop(void *opaque) { VirtIOBalloon *s = opaque; VirtIODevice *vdev = VIRTIO_DEVICE(s); qemu_spin_lock(); ... ==> s->free_page_report_status = FREE_PAGE_REPORT_S_STOP; ... qemu_spin_unlock(); } Without the lock, there are theoretical possibilities that assigning STOP below is overridden by START above. In that case,virtio_balloon_free_page_stop does not effectively stop virtio_balloon_poll_free_page_hints. I think this issue couldn't be solved by BHs. Best, Wei --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org