The virtio-rng linux driver can be stuck in virtio_read() on a wait_for_completion_killable() call if the virtio-rng device in QEMU doesn't provide data.
It's a problem, because virtio_read() is called from rng_get_data() with reading_mutex() held. The same mutex is taken by add_early_randomness() and hwrng_fillfn() and this brings to a hang during the boot sequence if the virtio-rng driver is builtin. Moreover, another lock is taken (rng_mutex) when the hwrng driver wants to switch the RNG device or the user tries to unplug the virtio-rng PCI card, and this can hang too because the virtio-rng driver is only able to release the card if the virtio-rng device sends back the virtqueue element. # echo -n virtio_rng.1 > /sys/class/misc/hw_random/rng_current [ 240.165234] INFO: task kworker/u2:1:34 blocked for more than 120 seconds. [ 240.165961] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 240.166708] kworker/u2:1 D ffffffffb86b85a8 0 34 2 0x00000000 [ 240.166714] Workqueue: kacpi_hotplug acpi_hotplug_work_fn [ 240.166716] ffffa0e8f3c0b890 0000000000000046 ffffa0e8f3c00000 ffffa0e8f3c0bfd8 [ 240.166717] ffffa0e8f3c0bfd8 ffffa0e8f3c0bfd8 ffffa0e8f3c00000 ffffffffb86b85a0 [ 240.166719] ffffffffb86b85a4 ffffa0e8f3c00000 00000000ffffffff ffffffffb86b85a8 [ 240.166720] Call Trace: [ 240.166725] [<ffffffffb82a61c9>] schedule_preempt_disabled+0x29/0x70 [ 240.166727] [<ffffffffb82a40f7>] __mutex_lock_slowpath+0xc7/0x1d0 [ 240.166728] [<ffffffffb82a350f>] mutex_lock+0x1f/0x2f [ 240.166730] [<ffffffffb8022b52>] hwrng_register+0x32/0x1d0 [ 240.166733] [<ffffffffc07fa149>] virtrng_scan+0x19/0x30 [virtio_rng] [ 240.166744] [<ffffffffc03108db>] virtio_dev_probe+0x1eb/0x290 [virtio] [ 240.166746] [<ffffffffb803d6e5>] driver_probe_device+0x145/0x3c0 ... In some case, the QEMU RNG backend is not able to provide data, and the virtio-rng device is not aware of that: - with rng-random using /dev/random and no entropy is available, - with rng-egd started with a socket in "server,nowait" mode and no daemon connected, - with rng-egd and an egd daemon that is not providing enough data, - ... To release the locks regularly, this patch adds a watchdog in QEMU virtio-rng device that sends back to the guest the virtqueue buffer with a 0 byte payload. This case is expected and correctly managed by the hwrng core. Signed-off-by: Laurent Vivier <lviv...@redhat.com> --- hw/virtio/virtio-rng.c | 23 +++++++++++++++++++++++ include/hw/virtio/virtio-rng.h | 1 + 2 files changed, 24 insertions(+) diff --git a/hw/virtio/virtio-rng.c b/hw/virtio/virtio-rng.c index 30493a258622..173ecd370c0e 100644 --- a/hw/virtio/virtio-rng.c +++ b/hw/virtio/virtio-rng.c @@ -19,6 +19,8 @@ #include "qom/object_interfaces.h" #include "trace.h" +#define VIRTIO_RNG_WATCHDOG_MS 500 + static bool is_guest_ready(VirtIORNG *vrng) { VirtIODevice *vdev = VIRTIO_DEVICE(vrng); @@ -38,6 +40,21 @@ static size_t get_request_size(VirtQueue *vq, unsigned quota) return in; } +static void watchdog(void *opaque) +{ + VirtIORNG *vrng = opaque; + VirtIODevice *vdev = VIRTIO_DEVICE(vrng); + VirtQueueElement *elem; + + /* wake up driver */ + elem = virtqueue_pop(vrng->vq, sizeof(VirtQueueElement)); + if (!elem) { + return; + } + virtqueue_push(vrng->vq, elem, 0); + virtio_notify(vdev, vrng->vq); +} + static void virtio_rng_process(VirtIORNG *vrng); /* Send data from a char device over to the guest */ @@ -98,6 +115,9 @@ static void virtio_rng_process(VirtIORNG *vrng) return; } + timer_mod(vrng->watchdog_timer, + qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) + VIRTIO_RNG_WATCHDOG_MS); + if (vrng->activate_timer) { timer_mod(vrng->rate_limit_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) + vrng->conf.period_ms); @@ -222,6 +242,7 @@ static void virtio_rng_device_realize(DeviceState *dev, Error **errp) vrng->vq = virtio_add_queue(vdev, 8, handle_input); vrng->quota_remaining = vrng->conf.max_bytes; + vrng->watchdog_timer = timer_new_ms(QEMU_CLOCK_VIRTUAL, watchdog, vrng); vrng->rate_limit_timer = timer_new_ms(QEMU_CLOCK_VIRTUAL, check_rate_limit, vrng); vrng->activate_timer = true; @@ -236,6 +257,8 @@ static void virtio_rng_device_unrealize(DeviceState *dev, Error **errp) VirtIORNG *vrng = VIRTIO_RNG(dev); qemu_del_vm_change_state_handler(vrng->vmstate); + timer_del(vrng->watchdog_timer); + timer_free(vrng->watchdog_timer); timer_del(vrng->rate_limit_timer); timer_free(vrng->rate_limit_timer); virtio_cleanup(vdev); diff --git a/include/hw/virtio/virtio-rng.h b/include/hw/virtio/virtio-rng.h index 922dce7caccf..05d6b0e7d881 100644 --- a/include/hw/virtio/virtio-rng.h +++ b/include/hw/virtio/virtio-rng.h @@ -42,6 +42,7 @@ typedef struct VirtIORNG { /* We purposefully don't migrate this state. The quota will reset on the * destination as a result. Rate limiting is host state, not guest state. */ + QEMUTimer *watchdog_timer; QEMUTimer *rate_limit_timer; int64_t quota_remaining; bool activate_timer; -- 2.21.0