Commit 331bf905 authored by Chris Wilson's avatar Chris Wilson
Browse files

drm/i915/gt: Mark the execlists->active as the primary volatile access 



Since we want to do a lockless read of the current active request, and
that request is written to by process_csb also without serialisation, we
need to instruct gcc to take care in reading the pointer itself.

Otherwise, we have observed execlists_active() to report 0x40.

[ 2400.760381] igt/para-4098    1..s. 2376479300us : process_csb: rcs0 cs-irq head=3, tail=4
[ 2400.760826] igt/para-4098    1..s. 2376479303us : process_csb: rcs0 csb[4]: status=0x00000001:0x00000000
[ 2400.761271] igt/para-4098    1..s. 2376479306us : trace_ports: rcs0: promote { b9c59:2622, b9c55:2624 }
[ 2400.761726] igt/para-4097    0d... 2376479311us : __i915_schedule: rcs0: -2147483648->3, inflight:0000000000000040, rq:ffff888208c1e940

which is impossible!

The answer is that as we keep the existing execlists->active pointing
into the array as we copy over that array, the unserialised read may see
a partial pointer value.

Fixes: df403069 ("drm/i915/execlists: Lift process_csb() out of the irq-off spinlock")
Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: default avatarMika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191125094318.1630806-1-chris@chris-wilson.co.uk
parent bae21dac

drivers/gpu/drm/i915/gt/intel_engine.h

+1 −3
Original line number Diff line number Diff line
@@ -100,9 +100,7 @@ execlists_num_ports(const struct intel_engine_execlists * const execlists) 
static inline struct i915_request *

execlists_active(const struct intel_engine_execlists *execlists)

{

	GEM_BUG_ON(execlists->active - execlists->inflight >

		   execlists_num_ports(execlists));

	return READ_ONCE(*execlists->active);

	return *READ_ONCE(execlists->active);

}



static inline void

drivers/gpu/drm/i915/gt/intel_lrc.c

+17 −10
Original line number Diff line number Diff line
@@ -2013,6 +2013,9 @@ cancel_port_requests(struct intel_engine_execlists * const execlists) 
		execlists_schedule_out(rq);

	memset(execlists->pending, 0, sizeof(execlists->pending));



	/* Mark the end of active before we overwrite *active */

	WRITE_ONCE(execlists->active, execlists->pending);



	for (port = execlists->active; (rq = *port); port++)

		execlists_schedule_out(rq);

	execlists->active =
@@ -2169,23 +2172,27 @@ static void process_csb(struct intel_engine_cs *engine) 
		else

			promote = gen8_csb_parse(execlists, buf + 2 * head);

		if (promote) {

			struct i915_request * const *old = execlists->active;



			/* Point active to the new ELSP; prevent overwriting */

			WRITE_ONCE(execlists->active, execlists->pending);

			set_timeslice(engine);



			if (!inject_preempt_hang(execlists))

				ring_set_paused(engine, 0);



			/* cancel old inflight, prepare for switch */

			trace_ports(execlists, "preempted", execlists->active);

			while (*execlists->active)

				execlists_schedule_out(*execlists->active++);

			trace_ports(execlists, "preempted", old);

			while (*old)

				execlists_schedule_out(*old++);



			/* switch pending to inflight */

			GEM_BUG_ON(!assert_pending_valid(execlists, "promote"));

			execlists->active =

			WRITE_ONCE(execlists->active,

				   memcpy(execlists->inflight,

					  execlists->pending,

					  execlists_num_ports(execlists) *

				       sizeof(*execlists->pending));



			set_timeslice(engine);

					  sizeof(*execlists->pending)));



			WRITE_ONCE(execlists->pending[0], NULL);

		} else {

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