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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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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