[bug report] vhost_net: basic polling support

From: Dan Carpenter <dan.carpenter@oracle.com>
To: jasowang@redhat.com
Cc: virtualization@lists.linux-foundation.org
Subject: [bug report] vhost_net: basic polling support
Date: Wed, 11 Aug 2021 15:13:44 +0300	[thread overview]
Message-ID: <20210811121344.GA14245@kili> (raw)

Hello Jason Wang,

The patch 030881372460: "vhost_net: basic polling support" from Mar
4, 2016, leads to the following
Smatch static checker warning:

	drivers/vhost/vhost.c:2565 vhost_new_msg()
	warn: sleeping in atomic context

vers/vhost/net.c
   509  static void vhost_net_busy_poll(struct vhost_net *net,
   510                                  struct vhost_virtqueue *rvq,
   511                                  struct vhost_virtqueue *tvq,
   512                                  bool *busyloop_intr,
   513                                  bool poll_rx)
   514  {
   515          unsigned long busyloop_timeout;
   516          unsigned long endtime;
   517          struct socket *sock;
   518          struct vhost_virtqueue *vq = poll_rx ? tvq : rvq;
   519  
   520          /* Try to hold the vq mutex of the paired virtqueue. We can't
   521           * use mutex_lock() here since we could not guarantee a
   522           * consistenet lock ordering.
   523           */
   524          if (!mutex_trylock(&vq->mutex))
   525                  return;
   526  
   527          vhost_disable_notify(&net->dev, vq);
   528          sock = vhost_vq_get_backend(rvq);
   529  
   530          busyloop_timeout = poll_rx ? rvq->busyloop_timeout:
   531                                       tvq->busyloop_timeout;
   532  
   533          preempt_disable();
                ^^^^^^^^^^^^^^^^^
This bumps the preemp_count.

I guess this is to disable page faults?

   534          endtime = busy_clock() + busyloop_timeout;
   535  
   536          while (vhost_can_busy_poll(endtime)) {
   537                  if (vhost_has_work(&net->dev)) {
   538                          *busyloop_intr = true;
   539                          break;
   540                  }
   541  
   542                  if ((sock_has_rx_data(sock) &&
   543                       !vhost_vq_avail_empty(&net->dev, rvq)) ||

The call tree from here to the GFP_KERNEL is very long...

vhost_vq_avail_empty()
-> vhost_get_avail_idx()
   -> __vhost_get_user()
      -> __vhost_get_user_slow()
         -> translate_desc()
            -> vhost_iotlb_miss vhost_new_msg()  <-- GFP_KERNEL

   544                      !vhost_vq_avail_empty(&net->dev, tvq))
   545                          break;
   546  
   547                  cpu_relax();
   548          }
   549  
   550          preempt_enable();
   551  
   552          if (poll_rx || sock_has_rx_data(sock))
   553                  vhost_net_busy_poll_try_queue(net, vq);

regards,
dan carpenter
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