Re: [Intel-gfx] [PATCH] [v2] drm/i915/pmu: avoid -Wmaybe-uninitialized warning

[Tvrtko Ursulin]

On 13/03/2018 16:19, Arnd Bergmann wrote:
> 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]

Hm, how does paravirt_types.h comes into the picture?

> 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.

Is it specific gcc version, specific options, or specific kernel config 
that this happens?

Strange that it hasn't been seen so far.

Regards,

Tvrtko

> Fixes: 1fe699e30113 ("drm/i915/pmu: Fix sleep under atomic in RC6 readout")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> v2: removed unused function argument, fixed 'break' statement.
> ---
>   drivers/gpu/drm/i915/i915_pmu.c | 25 ++++++++++---------------
>   1 file changed, 10 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> index 4bc7aefa9541..d6b9b6b5fb98 100644
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -412,10 +412,9 @@ 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;
>   	u64 val;
>   
>   	if (intel_runtime_pm_get_if_in_use(i915)) {
> @@ -428,18 +427,12 @@ static u64 get_rc6(struct drm_i915_private *i915, bool locked)
>   		 * previously.
>   		 */
>   
> -		if (!locked)
> -			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;
>   			i915->pmu.sample[__I915_SAMPLE_RC6].cur = val;
>   		} else {
>   			val = i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur;
>   		}
> -
> -		if (!locked)
> -			spin_unlock_irqrestore(&i915->pmu.lock, flags);
>   	} else {
>   		struct pci_dev *pdev = i915->drm.pdev;
>   		struct device *kdev = &pdev->dev;
> @@ -452,9 +445,6 @@ 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);
>   
>   		if (!i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur)
> @@ -470,9 +460,6 @@ static u64 get_rc6(struct drm_i915_private *i915, bool locked)
>   		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);
>   	}
>   
>   	return val;
> @@ -519,7 +506,15 @@ 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);
> +			if (!locked) {
> +				unsigned long flags;
> +
> +				spin_lock_irqsave(&i915->pmu.lock, flags);
> +				val = get_rc6(i915);
> +				spin_unlock_irqrestore(&i915->pmu.lock, flags);
> +			} else {
> +				val = get_rc6(i915);
> +			}
>   			break;
>   		}
>   	}