[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]
Subject: Re: [virtio-dev] Re: [PATCH v4 3/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT
On 03/15/2018 10:47 AM, Michael S. Tsirkin wrote:
On Thu, Mar 15, 2018 at 09:15:48AM +0800, Wei Wang wrote: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 <wei.w.wang@intel.com> Signed-off-by: Liang Li <liang.z.li@intel.com> CC: Michael S. Tsirkin <mst@redhat.com> CC: Dr. David Alan Gilbert <dgilbert@redhat.com> CC: Juan Quintela <quintela@redhat.com>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, WeiAren'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.I wonder whether that's enough. E.g. is there a race with guest trying to reset the device? That resets all VQs you know.
I think that's OK - we will call virtio_balloon_free_page_stop in the device reset function, and qemu_thread_join() in virtio_balloon_free_page_stop will wait till the optimization thread exits. That is, the reset will proceed after the optimization thread exits.
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, WeiDon't all BHs run under the BQL?
Actually the virtio_balloon_free_page_stop is called by the migration thread (instead of a BH). Even we guarantee the migration thread calls virtio_balloon_free_page_stop under BQL, the BQL is still too big for our case. Imagine this case: when the migration thread calls virtio_balloon_free_page_stop to stop the reporting, it blocks by BQL as virtio_balloon_poll_free_page_hints is in progress with BQL held, and the migration thread won't proceed untill virtio_balloon_poll_free_page_hints exits (i.e. getting all the hints). I think this isn't our intention - we basically want the migration thread to stop the guest reporting immediately.
So I think the small lock above is better (it locks for only one hint). Best, Wei
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]