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: df403069029d ("drm/i915/execlists: Lift process_csb() out of the irq-off spinlock")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/gt/intel_engine.h | 4 +---
drivers/gpu/drm/i915/gt/intel_lrc.c | 24 ++++++++++++++----------
2 files changed, 15 insertions(+), 13 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/intel_engine.h b/drivers/gpu/drm/i915/gt/intel_engine.h
index bc3b72bfa9e3..01765a7ec18f 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine.h
@@ -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
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 0e2065a13f24..0d0dca3d6724 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -2169,23 +2169,27 @@ 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 =
- memcpy(execlists->inflight,
- execlists->pending,
- execlists_num_ports(execlists) *
- sizeof(*execlists->pending));
-
- set_timeslice(engine);
+ WRITE_ONCE(execlists->active,
+ memcpy(execlists->inflight,
+ execlists->pending,
+ execlists_num_ports(execlists) *
+ sizeof(*execlists->pending)));
WRITE_ONCE(execlists->pending[0], NULL);
} else {
--
2.24.0
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