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re: kern/54495: stopping axe(4) may locks up if NET_MPSAFE is enabled

actually, i see how usbnet_tick triggers this.  some of
the driver did this, and i copied it into all, but i see
how it is espcially bad for axe(4) -- it has many many
more usbd_delay_us() calls compared to most of the other
usbnet converted drivers.

i can see a few ideas for fixes.

- remove any access to adaptive locks in softint context,
  which comes down to don't take the lock in usbnet_tick().

- arrange to have the un_lock passed to kpause so it is
  dropped and re-taken during kpause.

- push *all* the work in usbnet_tick() into the
  usbnet_tick_task() (hm, this may also be #1.)

i think the first idea is best and simplest.  i suspect
that it needs to be written like;

        struct usbnet_private * const unp = un->un_pri;

        if (unp != NULL && !unp->unp_stopping && !unp->unp_dying) {
                /* Perform periodic stuff in process context */
                usb_add_task(un->un_udev, &unp->unp_ticktask, USB_TASKQ_DRIVER);

can you test?  my axe(4) is broken.

(i've got a few more fixes to check unp for NULL in a
tree i'm testing, thanks to a report from maxv about
upl(4) crash during detach, which is the only fixed
part so far, though the rest of them are more safety
than actually fixing bugs.)



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