From: yaozhenguo <yaozhenguo1@gmail.com>

During the hot-unplugging of vhost-user-net type network cards,
the vhost_user_cleanup function may add the same rcu node to
the rcu linked list. The function call in this case is as follows:

vhost_user_cleanup
    ->vhost_user_host_notifier_remove
        ->call_rcu(n, vhost_user_host_notifier_free, rcu);
    ->g_free_rcu(n, rcu);

When this happens, QEMU will abort in try_dequeue:

if (head == &dummy && qatomic_mb_read(&tail) == &dummy.next) {
    abort();
}

backtrace is as follows:
0  __pthread_kill_implementation () at /usr/lib64/libc.so.6
1  raise () at /usr/lib64/libc.so.6
2  abort () at /usr/lib64/libc.so.6
3  try_dequeue () at ../util/rcu.c:235
4  call_rcu_thread (0) at ../util/rcu.c:288
5  qemu_thread_start (0) at ../util/qemu-thread-posix.c:541
6  start_thread () at /usr/lib64/libc.so.6
7  clone3 () at /usr/lib64/libc.so.6

The reason for the abort is that adding two identical nodes to
the rcu linked list will cause the rcu linked list to become a ring,
but when the dummy node is added after the two identical nodes,
the ring is opened. But only one node is added to list with
rcu_call_count added twice. This will cause rcu try_dequeue abort.

This happens when n->addr != 0. In some scenarios, this does happen.
For example, this situation will occur when using a 32-queue DPU
vhost-user-net type network card for hot-unplug testing, because
VhostUserHostNotifier->addr will be cleared during the processing of
VHOST_USER_BACKEND_VRING_HOST_NOTIFIER_MSG. However,it is asynchronous,
so we cannot guarantee that VhostUserHostNotifier->addr is zero in
vhost_user_cleanup. Therefore, it is necessary to merge g_free_rcu
and vhost_user_host_notifier_free into one rcu node.

Fixes: 503e355465 ("virtio/vhost-user: dynamically assign VhostUserHostNotifiers")
Signed-off-by: yaozhenguo <yaozhenguo@jd.com>
Message-Id: <20241011102913.45582-1-yaozhenguo@jd.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/hw/virtio/vhost-user.h |  1 +
 hw/virtio/vhost-user.c         | 46 +++++++++++++++++++---------------
 2 files changed, 27 insertions(+), 20 deletions(-)

diff --git a/include/hw/virtio/vhost-user.h b/include/hw/virtio/vhost-user.h
index 324cd8663a..9a3f238b43 100644
--- a/include/hw/virtio/vhost-user.h
+++ b/include/hw/virtio/vhost-user.h
@@ -54,6 +54,7 @@ typedef struct VhostUserHostNotifier {
     void *addr;
     void *unmap_addr;
     int idx;
+    bool destroy;
 } VhostUserHostNotifier;
 
 /**
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 00561daa06..d1b0893b4d 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -1185,9 +1185,16 @@ static int vhost_user_set_vring_num(struct vhost_dev *dev,
 
 static void vhost_user_host_notifier_free(VhostUserHostNotifier *n)
 {
-    assert(n && n->unmap_addr);
-    munmap(n->unmap_addr, qemu_real_host_page_size());
-    n->unmap_addr = NULL;
+    if (n->unmap_addr) {
+        munmap(n->unmap_addr, qemu_real_host_page_size());
+        n->unmap_addr = NULL;
+    }
+    if (n->destroy) {
+        memory_region_transaction_begin();
+        object_unparent(OBJECT(&n->mr));
+        memory_region_transaction_commit();
+        g_free(n);
+    }
 }
 
 /*
@@ -1195,17 +1202,28 @@ static void vhost_user_host_notifier_free(VhostUserHostNotifier *n)
  * under rcu.
  */
 static void vhost_user_host_notifier_remove(VhostUserHostNotifier *n,
-                                            VirtIODevice *vdev)
+                                            VirtIODevice *vdev, bool destroy)
 {
+    /*
+     * if destroy == false and n->addr == NULL, we have nothing to do.
+     * so, just return.
+     */
+    if (!n || (!destroy && !n->addr)) {
+        return;
+    }
+
     if (n->addr) {
         if (vdev) {
+            memory_region_transaction_begin();
             virtio_queue_set_host_notifier_mr(vdev, n->idx, &n->mr, false);
+            memory_region_transaction_commit();
         }
         assert(!n->unmap_addr);
         n->unmap_addr = n->addr;
         n->addr = NULL;
-        call_rcu(n, vhost_user_host_notifier_free, rcu);
     }
+    n->destroy = destroy;
+    call_rcu(n, vhost_user_host_notifier_free, rcu);
 }
 
 static int vhost_user_set_vring_base(struct vhost_dev *dev,
@@ -1279,9 +1297,7 @@ static int vhost_user_get_vring_base(struct vhost_dev *dev,
     struct vhost_user *u = dev->opaque;
 
     VhostUserHostNotifier *n = fetch_notifier(u->user, ring->index);
-    if (n) {
-        vhost_user_host_notifier_remove(n, dev->vdev);
-    }
+    vhost_user_host_notifier_remove(n, dev->vdev, false);
 
     ret = vhost_user_write(dev, &msg, NULL, 0);
     if (ret < 0) {
@@ -1562,7 +1578,7 @@ static int vhost_user_backend_handle_vring_host_notifier(struct vhost_dev *dev,
      * new mapped address.
      */
     n = fetch_or_create_notifier(user, queue_idx);
-    vhost_user_host_notifier_remove(n, vdev);
+    vhost_user_host_notifier_remove(n, vdev, false);
 
     if (area->u64 & VHOST_USER_VRING_NOFD_MASK) {
         return 0;
@@ -2736,15 +2752,7 @@ static int vhost_user_set_inflight_fd(struct vhost_dev *dev,
 static void vhost_user_state_destroy(gpointer data)
 {
     VhostUserHostNotifier *n = (VhostUserHostNotifier *) data;
-    if (n) {
-        vhost_user_host_notifier_remove(n, NULL);
-        object_unparent(OBJECT(&n->mr));
-        /*
-         * We can't free until vhost_user_host_notifier_remove has
-         * done it's thing so schedule the free with RCU.
-         */
-        g_free_rcu(n, rcu);
-    }
+    vhost_user_host_notifier_remove(n, NULL, true);
 }
 
 bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp)
@@ -2765,9 +2773,7 @@ void vhost_user_cleanup(VhostUserState *user)
     if (!user->chr) {
         return;
     }
-    memory_region_transaction_begin();
     user->notifiers = (GPtrArray *) g_ptr_array_free(user->notifiers, true);
-    memory_region_transaction_commit();
     user->chr = NULL;
 }
 
-- 
MST