ast: cursor flashing softlockups

[Dann Frazier]

Hi,
 I'm observing a soft lockup issue w/ the ASPEED controller on an
arm64 server platform. This was originally seen on Ubuntu's 4.4
kernel, but it is reproducible w/ vanilla 4.6-rc7 as well.

[   32.792656] NMI watchdog: BUG: soft lockup - CPU#38 stuck for 22s!
[swapper/38:0]

I observe this just once each time I boot into debian-installer (I'm
using a serial console, but the ast module gets loaded during
startup).

perf shows that the CPU caught by the NMI is typically in code
updating the cursor timer:

-   16.92%  swapper  [kernel.kallsyms]      [k] _raw_spin_unlock_irqrestore
   - _raw_spin_unlock_irqrestore
      + 16.87% mod_timer
      + 0.05% cursor_timer_handler
-   12.15%  swapper  [kernel.kallsyms]      [k] queue_work_on
   - queue_work_on
      + 12.00% cursor_timer_handler
      + 0.15% call_timer_fn
+   10.98%  swapper  [kernel.kallsyms]      [k] run_timer_softirq
-    2.23%  swapper  [kernel.kallsyms]      [k] mod_timer
   - mod_timer
      + 1.97% cursor_timer_handler
      + 0.26% call_timer_fn

During the same period, I can see that another CPU is actively
executing the timer function:

-   42.18%  kworker/u96:2  [kernel.kallsyms]  [k] ww_mutex_unlock
   - ww_mutex_unlock
      - 40.70% ast_dirty_update
           ast_imageblit
           soft_cursor
           bit_cursor
           fb_flashcursor
           process_one_work
           worker_thread
           kthread
           ret_from_fork
      + 1.48% ast_imageblit
-   40.15%  kworker/u96:2  [kernel.kallsyms]  [k] __memcpy_toio
   - __memcpy_toio
      + 31.54% ast_dirty_update
      + 8.61% ast_imageblit

Using the graph function tracer on fb_flashcursor(), I see that
ast_dirty_update usually takes around 60 us, in which it makes 16
calls to __memcpy_toio(). However, there is always one instance on
every boot of the installer where ast_dirty_update() takes ~98 *ms* to
complete, during which it makes 743 calls to __memcpy_toio(). While
that  doesn't directly account for the full 22s, I do wonder if that
maybe a smoking gun.

fyi, this is being tracked at: https://bugs.launchpad.net/bugs/1574814

  -dann