[PREEMPT_RT] bogus lockdep assert from i915 on v5.2-rt1

[PREEMPT_RT] bogus lockdep assert from i915 on v5.2-rt1

[Clark Williams]

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Thomas,

When looking at a problem on v5.2-rt1, I turned on lockdep and started getting warnings
from lockdep_assert_irqs_disabled() in the i915 driver. They're making these calls inside
a spin_lock_irqsave/spin_lock_irqrestore block, which of course doesn't fiddle with IRQs
when PREEMPT_RT is configured. The attached patch places the three calls inside
if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) blocks, so should avoid the bogus warning on RT.

Clark

-- 
The United States Coast Guard
Ruining Natural Selection since 1790

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From 8ed59c9195ce6fd338916b39155b738c557d0cab Mon Sep 17 00:00:00 2001
From: Clark Williams <williams@redhat.com>
Date: Sun, 14 Jul 2019 16:42:21 -0500
Subject: [PATCH 1/2] i915: avoid calling lockdep_assert_irqs_disabled on
 PREEMPT_RT_FULL

The PREEMPT_RT_FULL patchset keeps irqs enabled when operating on
spin_locks. Avoid this lockdep call on RT since in most cases it
will fail and WARN unnessesarily.

Signed-off-by: Clark Williams <williams@redhat.com>
---
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 832cb6b1e9bd..3eef6010ebf6 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -101,7 +101,8 @@ __dma_fence_signal__notify(struct dma_fence *fence)
 	struct dma_fence_cb *cur, *tmp;
 
 	lockdep_assert_held(fence->lock);
-	lockdep_assert_irqs_disabled();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
+		lockdep_assert_irqs_disabled();
 
 	list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
 		INIT_LIST_HEAD(&cur->node);
@@ -276,7 +277,8 @@ void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine)
 bool i915_request_enable_breadcrumb(struct i915_request *rq)
 {
 	lockdep_assert_held(&rq->lock);
-	lockdep_assert_irqs_disabled();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
+		lockdep_assert_irqs_disabled();
 
 	if (test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags)) {
 		struct intel_breadcrumbs *b = &rq->engine->breadcrumbs;
@@ -325,7 +327,8 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
 	struct intel_breadcrumbs *b = &rq->engine->breadcrumbs;
 
 	lockdep_assert_held(&rq->lock);
-	lockdep_assert_irqs_disabled();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
+		lockdep_assert_irqs_disabled();
 
 	/*
 	 * We must wait for b->irq_lock so that we know the interrupt handler
-- 
2.21.0

Re: [PREEMPT_RT] bogus lockdep assert from i915 on v5.2-rt1

[Thomas Gleixner]

Clark,

On Tue, 16 Jul 2019, Clark Williams wrote:

> Thomas,
> 
> When looking at a problem on v5.2-rt1, I turned on lockdep and started getting warnings
> from lockdep_assert_irqs_disabled() in the i915 driver. They're making these calls inside
> a spin_lock_irqsave/spin_lock_irqrestore block, which of course doesn't fiddle with IRQs
> when PREEMPT_RT is configured. The attached patch places the three calls inside
> if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) blocks, so should avoid the bogus warning on RT.

AFAICT, all three asserts are pointless because there are already the lock
held asserts and lockdep will warn anyway when these locks are taken with
interrupts enabled.

Thanks,

	tglx