[Intel-gfx] [PATCH] drm/i915/perf: fix perf stream opening lock

[Chris Wilson]

Quoting Lionel Landwerlin (2018-02-28 11:45:48)
> We're seeing on CI that some contexts don't have the programmed OA
> period timer that directs the OA unit on how often to write reports.
> 
> The issue is that we're not holding the drm lock from when we edit the
> context images down to when we set the exclusive_stream variable. This
> leaves a window for the deferred context allocation to call
> i915_oa_init_reg_state() that will not program the expected OA timer
> value, because we haven't set the exclusive_stream yet.

Thank you for this quick explanation. After staring at the diff to
figure out what race you saw, the succinct explanation of the window
between altering all previous contexts and marking up new contexts,
enlightens.

> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> Fixes: 701f8231a2f ("drm/i915/perf: prune OA configs")
> Cc: <stable at vger.kernel.org> # v4.14+
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102254
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103715
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103755

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris