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From: | Wei Wang |
Subject: | Re: [Qemu-devel] [virtio-dev] Re: [PATCH v4 3/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT |
Date: | Thu, 15 Mar 2018 09:15:48 +0800 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
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 <address@hidden> Signed-off-by: Liang Li <address@hidden> CC: Michael S. Tsirkin <address@hidden> CC: Dr. David Alan Gilbert <address@hidden> CC: Juan Quintela <address@hidden>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. 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
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