drm/i915: Disable tasklet scheduling across initial scheduling

During request submission, we call the engine->schedule() function so
that we may reorder the active requests as required for inheriting the
new request's priority. This may schedule several tasklets to run on the
local CPU, but we will need to schedule the tasklets again for the new
request. Delay all the local tasklets until the end, so that we only
have to process the queue just once.

v2: Beware PREEMPT_RCU, as then local_bh_disable() is then not a
superset of rcu_read_lock().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180507135731.10587-2-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index e4cf76ec14a6ac10555ce5d5bcbdb9179df1a13d..f336942229cf3e194512939917c62aaf5ff8c53d 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1110,12 +1110,11 @@ void __i915_request_add(struct i915_request *request, bool flush_caches)
         * decide whether to preempt the entire chain so that it is ready to
         * run at the earliest possible convenience.
         */
-       rcu_read_lock();
+       local_bh_disable();
+       rcu_read_lock(); /* RCU serialisation for set-wedged protection */
        if (engine->schedule)
                engine->schedule(request, &request->ctx->sched);
        rcu_read_unlock();
-
-       local_bh_disable();
        i915_sw_fence_commit(&request->submit);
        local_bh_enable(); /* Kick the execlists tasklet if just scheduled */