[PATCH] drm/i915/pmu: Work around compiler warnings on some kernel configs

* [PATCH] drm/i915/pmu: Work around compiler warnings on some kernel configs
@ 2018-03-14  8:05 Tvrtko Ursulin
  2018-03-14  8:15 ` ✗ Fi.CI.SPARSE: warning for " Patchwork
                   ` (3 more replies)
  0 siblings, 4 replies; 7+ messages in thread
From: Tvrtko Ursulin @ 2018-03-14  8:05 UTC (permalink / raw)
  To: Intel-gfx; +Cc: David Airlie, dri-devel, Rodrigo Vivi, intel-gfx

From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Arnd Bergman reports:
"""
The conditional spinlock confuses gcc into thinking the 'flags' value
might contain uninitialized data:

drivers/gpu/drm/i915/i915_pmu.c: In function '__i915_pmu_event_read':
arch/x86/include/asm/paravirt_types.h:573:3: error: 'flags' may be used uninitialized in this function [-Werror=maybe-uninitialized]

The code is correct, but it's easy to see how the compiler gets confused
here. This avoids the problem by pulling the lock outside of the function
into its only caller.
"""

On deeper look it seems this is caused by paravirt spinlocks
implementation when CONFIG_PARAVIRT_DEBUG is set, which by being
complicated, manages to convince gcc locked parameter can be changed
externally (impossible).

Work around it by removing the conditional locking parameters altogether.
(It was never the most elegant code anyway.)

Slight penalty we now pay is an additional irqsave spin lock/unlock cycle
on the event enable path. But since enable is not a fast path, that is
preferrable to the alternative solution which was doing MMIO under irqsave
spinlock.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 1fe699e30113 ("drm/i915/pmu: Fix sleep under atomic in RC6 readout")
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
---
 drivers/gpu/drm/i915/i915_pmu.c | 32 +++++++++++++-------------------
 1 file changed, 13 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index 4bc7aefa9541..11fb76bd3860 100644
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -412,7 +412,7 @@ static u64 __get_rc6(struct drm_i915_private *i915)
 	return val;
 }
 
-static u64 get_rc6(struct drm_i915_private *i915, bool locked)
+static u64 get_rc6(struct drm_i915_private *i915)
 {
 #if IS_ENABLED(CONFIG_PM)
 	unsigned long flags;
@@ -428,8 +428,7 @@ static u64 get_rc6(struct drm_i915_private *i915, bool locked)
 		 * previously.
 		 */
 
-		if (!locked)
-			spin_lock_irqsave(&i915->pmu.lock, flags);
+		spin_lock_irqsave(&i915->pmu.lock, flags);
 
 		if (val >= i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur) {
 			i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur = 0;
@@ -438,12 +437,10 @@ static u64 get_rc6(struct drm_i915_private *i915, bool locked)
 			val = i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur;
 		}
 
-		if (!locked)
-			spin_unlock_irqrestore(&i915->pmu.lock, flags);
+		spin_unlock_irqrestore(&i915->pmu.lock, flags);
 	} else {
 		struct pci_dev *pdev = i915->drm.pdev;
 		struct device *kdev = &pdev->dev;
-		unsigned long flags2;
 
 		/*
 		 * We are runtime suspended.
@@ -452,10 +449,8 @@ static u64 get_rc6(struct drm_i915_private *i915, bool locked)
 		 * on top of the last known real value, as the approximated RC6
 		 * counter value.
 		 */
-		if (!locked)
-			spin_lock_irqsave(&i915->pmu.lock, flags);
-
-		spin_lock_irqsave(&kdev->power.lock, flags2);
+		spin_lock_irqsave(&i915->pmu.lock, flags);
+		spin_lock(&kdev->power.lock);
 
 		if (!i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur)
 			i915->pmu.suspended_jiffies_last =
@@ -465,14 +460,13 @@ static u64 get_rc6(struct drm_i915_private *i915, bool locked)
 		      i915->pmu.suspended_jiffies_last;
 		val += jiffies - kdev->power.accounting_timestamp;
 
-		spin_unlock_irqrestore(&kdev->power.lock, flags2);
+		spin_unlock(&kdev->power.lock);
 
 		val = jiffies_to_nsecs(val);
 		val += i915->pmu.sample[__I915_SAMPLE_RC6].cur;
 		i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur = val;
 
-		if (!locked)
-			spin_unlock_irqrestore(&i915->pmu.lock, flags);
+		spin_unlock_irqrestore(&i915->pmu.lock, flags);
 	}
 
 	return val;
@@ -481,7 +475,7 @@ static u64 get_rc6(struct drm_i915_private *i915, bool locked)
 #endif
 }
 
-static u64 __i915_pmu_event_read(struct perf_event *event, bool locked)
+static u64 __i915_pmu_event_read(struct perf_event *event)
 {
 	struct drm_i915_private *i915 =
 		container_of(event->pmu, typeof(*i915), pmu.base);
@@ -519,7 +513,7 @@ static u64 __i915_pmu_event_read(struct perf_event *event, bool locked)
 			val = count_interrupts(i915);
 			break;
 		case I915_PMU_RC6_RESIDENCY:
-			val = get_rc6(i915, locked);
+			val = get_rc6(i915);
 			break;
 		}
 	}
@@ -534,7 +528,7 @@ static void i915_pmu_event_read(struct perf_event *event)
 
 again:
 	prev = local64_read(&hwc->prev_count);
-	new = __i915_pmu_event_read(event, false);
+	new = __i915_pmu_event_read(event);
 
 	if (local64_cmpxchg(&hwc->prev_count, prev, new) != prev)
 		goto again;
@@ -584,14 +578,14 @@ static void i915_pmu_enable(struct perf_event *event)
 		engine->pmu.enable_count[sample]++;
 	}
 
+	spin_unlock_irqrestore(&i915->pmu.lock, flags);
+
 	/*
 	 * Store the current counter value so we can report the correct delta
 	 * for all listeners. Even when the event was already enabled and has
 	 * an existing non-zero value.
 	 */
-	local64_set(&event->hw.prev_count, __i915_pmu_event_read(event, true));
-
-	spin_unlock_irqrestore(&i915->pmu.lock, flags);
+	local64_set(&event->hw.prev_count, __i915_pmu_event_read(event));
 }
 
 static void i915_pmu_disable(struct perf_event *event)
-- 
2.14.1

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