[Intel-gfx] [PATCH 6/6] drm/i915: Only call tasklet_kill() on the first prepare_reset

[Chris Wilson]

Quoting Chris Wilson (2018-03-07 13:42:26)
> tasklet_kill() will spin waiting for the current tasklet to be executed.
> However, if tasklet_disable() has been called, then the tasklet is never
> executed but permanently put back onto the runlist until
> tasklet_enable() is called. Ergo, we cannot use tasklet_kill() inside a
> disable/enable pair. This is the case when we call set-wedge from inside
> i915_reset(), and another request was submitted to us concurrent to the
> reset.
> 
> Fixes: 963ddd63c314 ("drm/i915: Suspend submission tasklets around wedging")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 3b44952e089f..e75af06904b7 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2941,7 +2941,8 @@ i915_gem_reset_prepare_engine(struct intel_engine_cs *engine)
>          * Turning off the execlists->tasklet until the reset is over
>          * prevents the race.
>          */
> -       tasklet_kill(&engine->execlists.tasklet);
> +       if (!atomic_read(&engine->execlists.tasklet.count))
> +               tasklet_kill(&engine->execlists.tasklet);

Note this is racy; two separate atomic operations. The only race we have
is with set-wedge vs reset, which is a rare and already contentious
issue. The upside of preventing the lockup is that we don't lose the
machine.

+        * Note that this needs to be a single atomic operation on the
+        * tasklet (flush existing tasks, prevent new tasks) to prevent
+        * a race between reset and set-wedged. It is not, so we do the best
+        * we can atm and make sure we don't lock the machine up in the more
+        * common case of recursing calling set-wedged from inside i915_reset.
-Chris