drm/i915: Add a strong mb to resetting the has-CS-interrupt bit

After a reset, the state of the CSB registers are scrubbed and not valid
until a powercontext is reloaded. We only know when a powercontext has
been reloaded once we see a CS-interrupt, before then we must ignore the
CSB registers within the execlists_submission_tasklet. However, glk is
sporadically dying with an illegal CSB pointer value (both in the HSWP
and mmio) suggesting that it is running with the CS-interrupt bit set
before the powercontext has been reloaded. Make sure the clearing of
that bit is serialised on reset with the re-enabling of the tasklet.

References: https://bugs.freedesktop.org/show_bug.cgi?id=104262
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171219090110.11153-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -3089,7 +3089,12 @@ i915_gem_reset_request(struct intel_engine_cs *engine,
 void i915_gem_reset_engine(struct intel_engine_cs *engine,
 			   struct drm_i915_gem_request *request)
 {
-	engine->irq_posted = 0;
+	/*
+	 * Make sure this write is visible before we re-enable the interrupt
+	 * handlers on another CPU, as tasklet_enable() resolves to just
+	 * a compiler barrier which is insufficient for our purpose here.
+	 */
+	smp_store_mb(engine->irq_posted, 0);
 
 	if (request)
 		request = i915_gem_reset_request(engine, request);