Hi Dan:
On Wed, Aug 11, 2021 at 8:14 PM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>
> 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?
No, it's intended since we will use busy_clock() which uses the per
cpu variable.
>
> 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
This won't happen in reality since:
We will make sure the IOTLB contains the translation for avail idx.
See vq_meta_prefetch() that will be called at the begining of
handle_tx() and handle_rx().
So it looks like a false positive to me.
Thanks
>
> 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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