Re: [Xen-devel] [v2 1/6] Port WARN_ON_ONCE() from Linux

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.05.18 at 22:23, <sameer.goel@xxxxxxxxxx> wrote:

> 
> On 05/24/2018 01:53 AM, Jan Beulich wrote:
>>>>> On 24.05.18 at 02:46, <sameer.goel@xxxxxxxxxx> wrote:
>>> Port WARN_ON_ONCE macro from Linux.
>> In such a case you should justify adjustments you've made:
> I can add more details, but have mostly just changed variable names. The 
> macro is self explanatory.
> 
> Should I just change this to: "Define WARN_ON_ONCE macro to mirror LInux 
> functionality"

That would seem better to me.

>>> --- a/xen/include/xen/lib.h
>>> +++ b/xen/include/xen/lib.h
>>> @@ -11,6 +11,19 @@
>>>   #define BUG_ON(p)  do { if (unlikely(p)) BUG();  } while (0)
>>>   #define WARN_ON(p) do { if (unlikely(p)) WARN(); } while (0)
>>>   
>>> +#define WARN_ON_ONCE(p)                               \
>>> +({                                                    \
>>> +    static bool __section(".data.unlikely") warned;   \
>> Linux uses .data.once. That or .data.cold would seem better to me than
>> .data.unlikely.
> I guess there is not reason to keep this in a specific section. I'll 
> just go ahead and remove the section here?

There certainly is a reason: We don't want such variables to sit in the
middle of an otherwise frequently accessed cache line. Hence the "cold"
part of the suggested alternatives name.

Jan



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