After assigning ourselves as the new bottom-half, we must perform a
cursory check to prevent a missed interrupt. Either we miss the interrupt
whilst programming the hardware, or if there was a previous waiter (for
a later seqno) they may be woken instead of us (due to the inherent race
in the unlocked read of b->tasklet in the irq handler) and so we miss the
wake up.
Spotted-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Fixes: 688e6c725816 ("drm/i915: Slaughter the thundering... herd")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/intel_breadcrumbs.c | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 009d6e121767..6fcbb52e50fb 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -65,7 +65,7 @@ static void irq_disable(struct intel_engine_cs *engine)
engine->irq_posted = false;
}
-static bool __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
+static void __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
{
struct intel_engine_cs *engine =
container_of(b, struct intel_engine_cs, breadcrumbs);
@@ -73,7 +73,7 @@ static bool __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
assert_spin_locked(&b->lock);
if (b->rpm_wakelock)
- return false;
+ return;
/* Since we are waiting on a request, the GPU should be busy
* and should have its own rpm reference. For completeness,
@@ -93,8 +93,6 @@ static bool __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
if (!b->irq_enabled ||
test_bit(engine->id, &i915->gpu_error.missed_irq_rings))
mod_timer(&b->fake_irq, jiffies + 1);
-
- return engine->irq_posted;
}
static void __intel_breadcrumbs_disable_irq(struct intel_breadcrumbs *b)
@@ -233,7 +231,15 @@ static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
GEM_BUG_ON(rb_first(&b->waiters) != &wait->node);
b->first_wait = wait;
smp_store_mb(b->tasklet, wait->tsk);
- first = __intel_breadcrumbs_enable_irq(b);
+ /* After assigning ourselves as the new bottom-half, we must
+ * perform a cursory check to prevent a missed interrupt.
+ * Either we miss the interrupt whilst programming the hardware,
+ * or if there was a previous waiter (for a later seqno) they
+ * may be woken instead of us (due to the inherent race
+ * in the unlocked read of b->tasklet in the irq handler) and
+ * so we miss the wake up.
+ */
+ __intel_breadcrumbs_enable_irq(b);
}
GEM_BUG_ON(!b->tasklet);
GEM_BUG_ON(!b->first_wait);
--
2.8.1
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