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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: df40306902 ("drm/i915/execlists: Lift process_csb() out of the irq-off spinlock")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191125094318.1630806-1-chris@chris-wilson.co.uk
(cherry picked from commit 331bf90591)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
This commit is contained in:
Chris Wilson 2019-11-25 09:43:18 +00:00 committed by Joonas Lahtinen
parent bf201f5eda
commit 97f9af78f3
2 changed files with 18 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
drivers/gpu/drm/i915/gt/intel_engine.h
View File

@ -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

27
drivers/gpu/drm/i915/gt/intel_lrc.c
View File

@ -1943,6 +1943,9 @@ 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 =
@ -2099,23 +2102,27 @@ 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 {