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

Tvrtko Ursulin
2018-03-13T17:46:54Z

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@arndb.de>
> ---
> 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;
> }
> }
>
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