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

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

[Arnd Bergmann]

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.

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;
 		}
 	}
-- 
2.9.0

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

[Chris Wilson]

Quoting Arnd Bergmann (2018-03-13 16:19:31)
> 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.
> 
> Fixes: 1fe699e30113 ("drm/i915/pmu: Fix sleep under atomic in RC6 readout")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris
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✗ Fi.CI.SPARSE: warning for drm/i915/pmu: avoid -Wmaybe-uninitialized warning (rev2)

[Patchwork]

== Series Details ==

Series: drm/i915/pmu: avoid -Wmaybe-uninitialized warning (rev2)
URL   : https://patchwork.freedesktop.org/series/39857/
State : warning

== Summary ==

$ dim sparse origin/drm-tip
Commit: drm/i915/pmu: avoid -Wmaybe-uninitialized warning
-O:drivers/gpu/drm/i915/i915_pmu.c:442:47: warning: context imbalance in 'get_rc6' - unexpected unlock
+

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Re: ✗ Fi.CI.SPARSE: warning for drm/i915/pmu: avoid -Wmaybe-uninitialized warning (rev2)

[Tvrtko Ursulin]

On 13/03/2018 16:46, Patchwork wrote:
> == Series Details ==
> 
> Series: drm/i915/pmu: avoid -Wmaybe-uninitialized warning (rev2)
> URL   : https://patchwork.freedesktop.org/series/39857/
> State : warning
> 
> == Summary ==
> 
> $ dim sparse origin/drm-tip
> Commit: drm/i915/pmu: avoid -Wmaybe-uninitialized warning
> -O:drivers/gpu/drm/i915/i915_pmu.c:442:47: warning: context imbalance in 'get_rc6' - unexpected unlock
> +

Is sparse being run before the patch has been applied?!

Regards,

Tvrtko

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Re: ✗ Fi.CI.SPARSE: warning for drm/i915/pmu: avoid -Wmaybe-uninitialized warning (rev2)

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-03-13 17:41:59)
> 
> On 13/03/2018 16:46, Patchwork wrote:
> > == Series Details ==
> > 
> > Series: drm/i915/pmu: avoid -Wmaybe-uninitialized warning (rev2)
> > URL   : https://patchwork.freedesktop.org/series/39857/
> > State : warning
> > 
> > == Summary ==
> > 
> > $ dim sparse origin/drm-tip
> > Commit: drm/i915/pmu: avoid -Wmaybe-uninitialized warning
> > -O:drivers/gpu/drm/i915/i915_pmu.c:442:47: warning: context imbalance in 'get_rc6' - unexpected unlock
> > +
> 
> Is sparse being run before the patch has been applied?!

Sparse is definitely complaining about the apparent imbalance in
drm-tip. So the -+ does look like it means that warning was removed by
the patch.
-Chris
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✗ Fi.CI.BAT: warning for drm/i915/pmu: avoid -Wmaybe-uninitialized warning (rev2)

[Patchwork]

== Series Details ==

Series: drm/i915/pmu: avoid -Wmaybe-uninitialized warning (rev2)
URL   : https://patchwork.freedesktop.org/series/39857/
State : warning

== Summary ==

Series 39857v2 drm/i915/pmu: avoid -Wmaybe-uninitialized warning
https://patchwork.freedesktop.org/api/1.0/series/39857/revisions/2/mbox/

---- Possible new issues:

Test kms_busy:
        Subgroup basic-flip-a:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-flip-b:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-flip-c:
                skip       -> PASS       (fi-ivb-3770)
Test kms_cursor_legacy:
        Subgroup basic-busy-flip-before-cursor-atomic:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-busy-flip-before-cursor-legacy:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-flip-after-cursor-atomic:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-flip-after-cursor-legacy:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-flip-after-cursor-varying-size:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-flip-before-cursor-atomic:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-flip-before-cursor-legacy:
                skip       -> PASS       (fi-ivb-3770)
        Subgroup basic-flip-before-cursor-varying-size:
                skip       -> PASS       (fi-ivb-3770)
Test kms_force_connector_basic:
        Subgroup prune-stale-modes:
                pass       -> SKIP       (fi-ivb-3770)
Test kms_frontbuffer_tracking:
        Subgroup basic:
                skip       -> PASS       (fi-ivb-3770)

---- Known issues:

Test kms_chamelium:
        Subgroup dp-crc-fast:
                fail       -> PASS       (fi-kbl-7500u) fdo#103841
Test kms_pipe_crc_basic:
        Subgroup suspend-read-crc-pipe-b:
                pass       -> INCOMPLETE (fi-snb-2520m) fdo#103713
Test prime_vgem:
        Subgroup basic-fence-flip:
                skip       -> PASS       (fi-ivb-3770) fdo#104008

fdo#103841 https://bugs.freedesktop.org/show_bug.cgi?id=103841
fdo#103713 https://bugs.freedesktop.org/show_bug.cgi?id=103713
fdo#104008 https://bugs.freedesktop.org/show_bug.cgi?id=104008

fi-bdw-5557u     total:288  pass:267  dwarn:0   dfail:0   fail:0   skip:21  time:433s
fi-bdw-gvtdvm    total:288  pass:264  dwarn:0   dfail:0   fail:0   skip:24  time:434s
fi-blb-e6850     total:288  pass:223  dwarn:1   dfail:0   fail:0   skip:64  time:383s
fi-bsw-n3050     total:288  pass:242  dwarn:0   dfail:0   fail:0   skip:46  time:534s
fi-bwr-2160      total:288  pass:183  dwarn:0   dfail:0   fail:0   skip:105 time:299s
fi-bxt-dsi       total:288  pass:258  dwarn:0   dfail:0   fail:0   skip:30  time:511s
fi-bxt-j4205     total:288  pass:259  dwarn:0   dfail:0   fail:0   skip:29  time:510s
fi-byt-j1900     total:288  pass:253  dwarn:0   dfail:0   fail:0   skip:35  time:510s
fi-byt-n2820     total:288  pass:249  dwarn:0   dfail:0   fail:0   skip:39  time:496s
fi-cfl-8700k     total:288  pass:260  dwarn:0   dfail:0   fail:0   skip:28  time:411s
fi-cfl-s2        total:288  pass:262  dwarn:0   dfail:0   fail:0   skip:26  time:578s
fi-cnl-y3        total:288  pass:262  dwarn:0   dfail:0   fail:0   skip:26  time:592s
fi-elk-e7500     total:288  pass:229  dwarn:0   dfail:0   fail:0   skip:59  time:431s
fi-gdg-551       total:288  pass:179  dwarn:0   dfail:0   fail:1   skip:108 time:313s
fi-glk-1         total:288  pass:260  dwarn:0   dfail:0   fail:0   skip:28  time:533s
fi-hsw-4770      total:288  pass:261  dwarn:0   dfail:0   fail:0   skip:27  time:402s
fi-ilk-650       total:288  pass:228  dwarn:0   dfail:0   fail:0   skip:60  time:425s
fi-ivb-3520m     total:288  pass:259  dwarn:0   dfail:0   fail:0   skip:29  time:474s
fi-ivb-3770      total:288  pass:255  dwarn:0   dfail:0   fail:0   skip:33  time:430s
fi-kbl-7500u     total:288  pass:263  dwarn:1   dfail:0   fail:0   skip:24  time:473s
fi-kbl-7567u     total:288  pass:268  dwarn:0   dfail:0   fail:0   skip:20  time:466s
fi-kbl-r         total:288  pass:261  dwarn:0   dfail:0   fail:0   skip:27  time:518s
fi-pnv-d510      total:288  pass:222  dwarn:1   dfail:0   fail:0   skip:65  time:644s
fi-skl-6260u     total:288  pass:268  dwarn:0   dfail:0   fail:0   skip:20  time:438s
fi-skl-6600u     total:288  pass:261  dwarn:0   dfail:0   fail:0   skip:27  time:526s
fi-skl-6700hq    total:288  pass:262  dwarn:0   dfail:0   fail:0   skip:26  time:543s
fi-skl-6700k2    total:288  pass:264  dwarn:0   dfail:0   fail:0   skip:24  time:507s
fi-skl-6770hq    total:288  pass:268  dwarn:0   dfail:0   fail:0   skip:20  time:502s
fi-skl-guc       total:288  pass:260  dwarn:0   dfail:0   fail:0   skip:28  time:426s
fi-skl-gvtdvm    total:288  pass:265  dwarn:0   dfail:0   fail:0   skip:23  time:437s
fi-snb-2520m     total:245  pass:211  dwarn:0   dfail:0   fail:0   skip:33 
fi-snb-2600      total:288  pass:248  dwarn:0   dfail:0   fail:0   skip:40  time:406s
Blacklisted hosts:
fi-cfl-u         total:288  pass:262  dwarn:0   dfail:0   fail:0   skip:26  time:514s
fi-cnl-drrs      total:288  pass:257  dwarn:3   dfail:0   fail:0   skip:28  time:525s

874b86a759851707e26286c22062f6ccc526e46f drm-tip: 2018y-03m-13d-12h-36m-17s UTC integration manifest
dfa0bf3ec528 drm/i915/pmu: avoid -Wmaybe-uninitialized warning

== Logs ==

For more details see: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_8331/issues.html
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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@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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Re: [Intel-gfx] [PATCH] [v2] drm/i915/pmu: avoid -Wmaybe-uninitialized warning

[Arnd Bergmann]

On Tue, Mar 13, 2018 at 6:46 PM, Tvrtko Ursulin
<tvrtko.ursulin@linux.intel.com> wrote:
>
> 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?

spin_unlock_irqrestore() calls arch_local_irq_restore()

>> 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?

Not gcc version specific (same result with gcc-4.9 through 8, didn't test
earlier versions that are currently broken).

> Strange that it hasn't been seen so far.

It seems to be a relatively rare 'randconfig' combination. Looking at
the preprocessed sources, I find:

static u64 get_rc6(struct drm_i915_private *i915, bool locked)
{

 unsigned long flags;
 u64 val;

 if (intel_runtime_pm_get_if_in_use(i915)) {
  val = __get_rc6(i915);
  intel_runtime_pm_put(i915);
  if (!locked)
   do { do { ({ unsigned long __dummy; typeof(flags) __dummy2;
(void)(&__dummy == &__dummy2); 1; }); do { do { do { ({ unsigned long
__dummy; typeof(flags) __dummy2; (void)(&__dummy == &__dummy2); 1; });
flags = arch_local_irq_save(); } while (0); trace_hardirqs_off(); }
while (0); do { __asm__ __volatile__("": : :"memory"); do { (void)0;
(void)(spinlock_check(&i915->pmu.lock)); } while (0); } while (0); }
while (0); } while (0); } while (0);

  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;
  unsigned long flags2;
# 455 "/git/arm-soc/drivers/gpu/drm/i915/i915_pmu.c"
  if (!locked)
   do { do { ({ unsigned long __dummy; typeof(flags) __dummy2;
(void)(&__dummy == &__dummy2); 1; }); do { do { do { ({ unsigned long
__dummy; typeof(flags) __dummy2; (void)(&__dummy == &__dummy2); 1; });
flags = arch_local_irq_save(); } while (0); trace_hardirqs_off(); }
while (0); do { __asm__ __volatile__("": : :"memory"); do { (void)0;
(void)(spinlock_check(&i915->pmu.lock)); } while (0); } while (0); }
while (0); } while (0); } while (0);

  do { do { ({ unsigned long __dummy; typeof(flags2) __dummy2;
(void)(&__dummy == &__dummy2); 1; }); do { do { do { ({ unsigned long
__dummy; typeof(flags2) __dummy2; (void)(&__dummy == &__dummy2); 1;
}); flags2 = arch_local_irq_save(); } while (0); trace_hardirqs_off();
} while (0); do { __asm__ __volatile__("": : :"memory"); do { (void)0;
(void)(spinlock_check(&kdev->power.lock)); } while (0); } while (0); }
while (0); } while (0); } while (0);

  if (!i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur)
   i915->pmu.suspended_jiffies_last =
      kdev->power.suspended_jiffies;

  val = kdev->power.suspended_jiffies -
        i915->pmu.suspended_jiffies_last;
  val += jiffies - kdev->power.accounting_timestamp;

  spin_unlock_irqrestore(&kdev->power.lock, flags2);

  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;
}

so it seems that the spin_lock_irqsave() is completely inlined through
a macro while the unlock is not, and the lock contains a memory barrier
(among other things) that might tell the compiler that the state of the
'locked' flag could changed underneath it.

It could also be the problem that arch_local_irq_restore() uses
__builtin_expect() in  PVOP_TEST_NULL(op) when
CONFIG_PARAVIRT_DEBUG is enabled, see

static inline __attribute__((unused))
__attribute__((no_instrument_function))
__attribute__((no_instrument_function)) void
arch_local_irq_restore(unsigned long f)
{
 ({ unsigned long __eax = __eax, __edx = __edx, __ecx = __ecx;; do {
if (__builtin_expect(!!(pv_irq_ops.restore_fl.func == ((void *)0)),
0)) do { do { asm volatile("1:\t" ".byte 0x0f, 0x0b" "\n"
".pushsection __bug_table,\"aw\"\n" "2:\t" ".long " "1b" "\t#
bug_entry::bug_addr\n" "\t" ".long " "%c0" "\t# bug_entry::file\n"
"\t.word %c1" "\t# bug_entry::line\n" "\t.word %c2" "\t#
bug_entry::flags\n" "\t.org 2b+%c3\n" ".popsection" : : "i"
("/git/arm-soc/arch/x86/include/asm/paravirt.h"), "i" (783), "i" (0),
"i" (sizeof(struct bug_entry))); } while (0); do { ; asm volatile("");
__builtin_unreachable(); } while (0); } while (0); } while (0); asm
volatile("" "771:\n\t" "999:\n\t" ".pushsection
.discard.retpoline_safe\n\t" " " ".long" " " " 999b\n\t"
".popsection\n\t" "call *%c[paravirt_opptr];" "\n" "772:\n"
".pushsection .parainstructions,\"a\"\n" " " ".balign 4" " " "\n" " "
".long" " " " 771b\n" "  .byte " "%c[paravirt_typenum]" "\n" "  .byte
772b-771b\n" "  .short " "%c[paravirt_clobber]" "\n" ".popsection\n"
"" : "=a" (__eax), "=d" (__edx), "+r" (current_stack_pointer) :
[paravirt_typenum] "i" ((__builtin_offsetof(struct
paravirt_patch_template, pv_irq_ops.restore_fl.func) / sizeof(void
*))), [paravirt_opptr] "i" (&(pv_irq_ops.restore_fl.func)),
[paravirt_clobber] "i" (((1 << 0) | (1 << 2))), "a" ((unsigned
long)(f)) : "memory", "cc" ); });
}

this seems to frequently confuse gcc, and turning off that NULL check
avoids the warning as well.

If you want to analyze it further, see https://pastebin.com/T2yLRqU5
for the .config file, but I'm pretty sure this is a known problem with gcc
that happens to be very hard to fix.

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

[Tvrtko Ursulin]

On 13/03/2018 20:10, Arnd Bergmann wrote:
> On Tue, Mar 13, 2018 at 6:46 PM, Tvrtko Ursulin
> <tvrtko.ursulin@linux.intel.com> wrote:
>>
>> 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?
> 
> spin_unlock_irqrestore() calls arch_local_irq_restore()
> 
>>> 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?
> 
> Not gcc version specific (same result with gcc-4.9 through 8, didn't test
> earlier versions that are currently broken).
> 
>> Strange that it hasn't been seen so far.
> 
> It seems to be a relatively rare 'randconfig' combination. Looking at
> the preprocessed sources, I find:
> 
> static u64 get_rc6(struct drm_i915_private *i915, bool locked)
> {
> 
>   unsigned long flags;
>   u64 val;
> 
>   if (intel_runtime_pm_get_if_in_use(i915)) {
>    val = __get_rc6(i915);
>    intel_runtime_pm_put(i915);
>    if (!locked)
>     do { do { ({ unsigned long __dummy; typeof(flags) __dummy2;
> (void)(&__dummy == &__dummy2); 1; }); do { do { do { ({ unsigned long
> __dummy; typeof(flags) __dummy2; (void)(&__dummy == &__dummy2); 1; });
> flags = arch_local_irq_save(); } while (0); trace_hardirqs_off(); }
> while (0); do { __asm__ __volatile__("": : :"memory"); do { (void)0;
> (void)(spinlock_check(&i915->pmu.lock)); } while (0); } while (0); }
> while (0); } while (0); } while (0);
> 
>    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;
>    unsigned long flags2;
> # 455 "/git/arm-soc/drivers/gpu/drm/i915/i915_pmu.c"
>    if (!locked)
>     do { do { ({ unsigned long __dummy; typeof(flags) __dummy2;
> (void)(&__dummy == &__dummy2); 1; }); do { do { do { ({ unsigned long
> __dummy; typeof(flags) __dummy2; (void)(&__dummy == &__dummy2); 1; });
> flags = arch_local_irq_save(); } while (0); trace_hardirqs_off(); }
> while (0); do { __asm__ __volatile__("": : :"memory"); do { (void)0;
> (void)(spinlock_check(&i915->pmu.lock)); } while (0); } while (0); }
> while (0); } while (0); } while (0);
> 
>    do { do { ({ unsigned long __dummy; typeof(flags2) __dummy2;
> (void)(&__dummy == &__dummy2); 1; }); do { do { do { ({ unsigned long
> __dummy; typeof(flags2) __dummy2; (void)(&__dummy == &__dummy2); 1;
> }); flags2 = arch_local_irq_save(); } while (0); trace_hardirqs_off();
> } while (0); do { __asm__ __volatile__("": : :"memory"); do { (void)0;
> (void)(spinlock_check(&kdev->power.lock)); } while (0); } while (0); }
> while (0); } while (0); } while (0);
> 
>    if (!i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur)
>     i915->pmu.suspended_jiffies_last =
>        kdev->power.suspended_jiffies;
> 
>    val = kdev->power.suspended_jiffies -
>          i915->pmu.suspended_jiffies_last;
>    val += jiffies - kdev->power.accounting_timestamp;
> 
>    spin_unlock_irqrestore(&kdev->power.lock, flags2);
> 
>    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;
> }
> 
> so it seems that the spin_lock_irqsave() is completely inlined through
> a macro while the unlock is not, and the lock contains a memory barrier
> (among other things) that might tell the compiler that the state of the
> 'locked' flag could changed underneath it.

Ha, interesting. So it sounds more like us having to workaround a bug in 
the paravirt spinlock macros.

I think I would prefer a different solution, where we don't end up doing 
MMIO under irqsave spinlock. I'll send a patch.

Regards,

Tvrtko

> 
> It could also be the problem that arch_local_irq_restore() uses
> __builtin_expect() in  PVOP_TEST_NULL(op) when
> CONFIG_PARAVIRT_DEBUG is enabled, see
> 
> static inline __attribute__((unused))
> __attribute__((no_instrument_function))
> __attribute__((no_instrument_function)) void
> arch_local_irq_restore(unsigned long f)
> {
>   ({ unsigned long __eax = __eax, __edx = __edx, __ecx = __ecx;; do {
> if (__builtin_expect(!!(pv_irq_ops.restore_fl.func == ((void *)0)),
> 0)) do { do { asm volatile("1:\t" ".byte 0x0f, 0x0b" "\n"
> ".pushsection __bug_table,\"aw\"\n" "2:\t" ".long " "1b" "\t#
> bug_entry::bug_addr\n" "\t" ".long " "%c0" "\t# bug_entry::file\n"
> "\t.word %c1" "\t# bug_entry::line\n" "\t.word %c2" "\t#
> bug_entry::flags\n" "\t.org 2b+%c3\n" ".popsection" : : "i"
> ("/git/arm-soc/arch/x86/include/asm/paravirt.h"), "i" (783), "i" (0),
> "i" (sizeof(struct bug_entry))); } while (0); do { ; asm volatile("");
> __builtin_unreachable(); } while (0); } while (0); } while (0); asm
> volatile("" "771:\n\t" "999:\n\t" ".pushsection
> .discard.retpoline_safe\n\t" " " ".long" " " " 999b\n\t"
> ".popsection\n\t" "call *%c[paravirt_opptr];" "\n" "772:\n"
> ".pushsection .parainstructions,\"a\"\n" " " ".balign 4" " " "\n" ""
> ".long" " " " 771b\n" "  .byte " "%c[paravirt_typenum]" "\n" "  .byte
> 772b-771b\n" "  .short " "%c[paravirt_clobber]" "\n" ".popsection\n"
> "" : "=a" (__eax), "=d" (__edx), "+r" (current_stack_pointer) :
> [paravirt_typenum] "i" ((__builtin_offsetof(struct
> paravirt_patch_template, pv_irq_ops.restore_fl.func) / sizeof(void
> *))), [paravirt_opptr] "i" (&(pv_irq_ops.restore_fl.func)),
> [paravirt_clobber] "i" (((1 << 0) | (1 << 2))), "a" ((unsigned
> long)(f)) : "memory", "cc" ); });
> }
> 
> this seems to frequently confuse gcc, and turning off that NULL check
> avoids the warning as well.
> 
> If you want to analyze it further, see https://pastebin.com/T2yLRqU5
> for the .config file, but I'm pretty sure this is a known problem with gcc
> that happens to be very hard to fix.
> 
>         Arnd
> 
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