Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> 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
Where is this exact tracepoint? My grep skills are failing me.
>
> 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.
...otherwise we will see ?
Also, the 0x40 is bothering me as I didn't find the tracepoint. If we
only displayed pointer values, where did the offset appear.
>
> 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);
Yes this seems proper as we need apriori read before deferencing.
> }
>
> 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 we set the active to pending here...
> +
> 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)));
Why we rewrite it in here, is the pending moving beneath us?
-Mika
>
> WRITE_ONCE(execlists->pending[0], NULL);
> } else {
> --
> 2.24.0
>
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