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!
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 | 6 ++++--
2 files changed, 5 insertions(+), 5 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..1d0a3fbd4884 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -2177,6 +2177,10 @@ static void process_csb(struct intel_engine_cs *engine)
while (*execlists->active)
execlists_schedule_out(*execlists->active++);
+ /* Point active to the new ELSP; prevent overwriting */
+ WRITE_ONCE(execlists->active, execlists->pending);
+ set_timeslice(engine);
+
/* switch pending to inflight */
GEM_BUG_ON(!assert_pending_valid(execlists, "promote"));
execlists->active =
@@ -2185,8 +2189,6 @@ static void process_csb(struct intel_engine_cs *engine)
execlists_num_ports(execlists) *
sizeof(*execlists->pending));
- set_timeslice(engine);
-
WRITE_ONCE(execlists->pending[0], NULL);
} else {
GEM_BUG_ON(!*execlists->active);
--
2.24.0
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