From 75dcb4d790bbe5327169fd72b185960ca58e2fa6 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Tue, 4 Jul 2023 17:15:27 +0200 Subject: [PATCH] virtio-blk: fix host notifier issues during dataplane start/stop MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The main loop thread can consume 100% CPU when using --device virtio-blk-pci,iothread=. ppoll() constantly returns but reading virtqueue host notifiers fails with EAGAIN. The file descriptors are stale and remain registered with the AioContext because of bugs in the virtio-blk dataplane start/stop code. The problem is that the dataplane start/stop code involves drain operations, which call virtio_blk_drained_begin() and virtio_blk_drained_end() at points where the host notifier is not operational: - In virtio_blk_data_plane_start(), blk_set_aio_context() drains after vblk->dataplane_started has been set to true but the host notifier has not been attached yet. - In virtio_blk_data_plane_stop(), blk_drain() and blk_set_aio_context() drain after the host notifier has already been detached but with vblk->dataplane_started still set to true. I would like to simplify ->ioeventfd_start/stop() to avoid interactions with drain entirely, but couldn't find a way to do that. Instead, this patch accepts the fragile nature of the code and reorders it so that vblk->dataplane_started is false during drain operations. This way the virtio_blk_drained_begin() and virtio_blk_drained_end() calls don't touch the host notifier. The result is that virtio_blk_data_plane_start() and virtio_blk_data_plane_stop() have complete control over the host notifier and stale file descriptors are no longer left in the AioContext. This patch fixes the 100% CPU consumption in the main loop thread and correctly moves host notifier processing to the IOThread. Fixes: 1665d9326fd2 ("virtio-blk: implement BlockDevOps->drained_begin()") Reported-by: Lukáš Doktor Signed-off-by: Stefan Hajnoczi Tested-by: Lukas Doktor Message-id: 20230704151527.193586-1-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi --- hw/block/dataplane/virtio-blk.c | 53 +++++++++++++++++++-------------- 1 file changed, 31 insertions(+), 22 deletions(-) diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c index c227b39408..da36fcfd0b 100644 --- a/hw/block/dataplane/virtio-blk.c +++ b/hw/block/dataplane/virtio-blk.c @@ -219,13 +219,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev) memory_region_transaction_commit(); - /* - * These fields are visible to the IOThread so we rely on implicit barriers - * in aio_context_acquire() on the write side and aio_notify_accept() on - * the read side. - */ - s->starting = false; - vblk->dataplane_started = true; trace_virtio_blk_data_plane_start(s); old_context = blk_get_aio_context(s->conf->conf.blk); @@ -244,6 +237,18 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev) event_notifier_set(virtio_queue_get_host_notifier(vq)); } + /* + * These fields must be visible to the IOThread when it processes the + * virtqueue, otherwise it will think dataplane has not started yet. + * + * Make sure ->dataplane_started is false when blk_set_aio_context() is + * called above so that draining does not cause the host notifier to be + * detached/attached prematurely. + */ + s->starting = false; + vblk->dataplane_started = true; + smp_wmb(); /* paired with aio_notify_accept() on the read side */ + /* Get this show started by hooking up our callbacks */ if (!blk_in_drain(s->conf->conf.blk)) { aio_context_acquire(s->ctx); @@ -273,7 +278,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev) fail_guest_notifiers: vblk->dataplane_disabled = true; s->starting = false; - vblk->dataplane_started = true; return -ENOSYS; } @@ -327,19 +331,6 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev) aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s); } - aio_context_acquire(s->ctx); - - /* Wait for virtio_blk_dma_restart_bh() and in flight I/O to complete */ - blk_drain(s->conf->conf.blk); - - /* - * Try to switch bs back to the QEMU main loop. If other users keep the - * BlockBackend in the iothread, that's ok - */ - blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context(), NULL); - - aio_context_release(s->ctx); - /* * Batch all the host notifiers in a single transaction to avoid * quadratic time complexity in address_space_update_ioeventfds(). @@ -360,12 +351,30 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev) virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i); } + /* + * Set ->dataplane_started to false before draining so that host notifiers + * are not detached/attached anymore. + */ + vblk->dataplane_started = false; + + aio_context_acquire(s->ctx); + + /* Wait for virtio_blk_dma_restart_bh() and in flight I/O to complete */ + blk_drain(s->conf->conf.blk); + + /* + * Try to switch bs back to the QEMU main loop. If other users keep the + * BlockBackend in the iothread, that's ok + */ + blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context(), NULL); + + aio_context_release(s->ctx); + qemu_bh_cancel(s->bh); notify_guest_bh(s); /* final chance to notify guest */ /* Clean up guest notifier (irq) */ k->set_guest_notifiers(qbus->parent, nvqs, false); - vblk->dataplane_started = false; s->stopping = false; }