[RFC] perf: fix building for ARCv1

[Vineet Gupta]

On Thursday 29 October 2015 09:28 PM, Alexey Brodkin wrote:
> Hi Vineet,
>
> On Tue, 2015-10-20 at 10:45 +0000, Vineet Gupta wrote:
>> On Tuesday 20 October 2015 03:41 PM, Peter Zijlstra wrote:
>>>>> Can we use existing syscall(s) - again this is what our good old pthread library
>>>>> code did.
>>>>>
>>>>> static void __pthread_acquire(int * spinlock)
>>>>> {
>>>>>   int cnt = 0;
>>>>>   struct timespec tm;
>>>>>
>>>>>   READ_MEMORY_BARRIER();
>>>>>
>>>>>   while (testandset(spinlock)) {   <---- atomic EXchange
>>>>>     if (cnt < 50) {
>>>>>       sched_yield();
>>>>>       cnt++;
>>>>>     } else {
>>>>>       tm.tv_sec = 0;
>>>>>       tm.tv_nsec = 2000001;
>>>>>       nanosleep(&tm, ((void *)0));
>>>>>       cnt = 0;
>>>>>     }
>>>>>   }
>>> *shudder* that is quite horrible.
>>>
>>> This means all your 'atomics' are broken for anything SCHED_FIFO and the
>>> like. You simply _cannot_ run a realtime system.
>> The code above is from uClibc old threading library which we don't use anymore.
>> The NPTL version doesn't have all of this song-n-dance and relies on futexes. The
>> change we are talking about is only for the atomics in perf itself. I do
>> understand your POV though.
>>
>>> (also, for ACQUIRE you want the READ_MEMORY_BARRIER() _after_ the
>>> test-and-set control dependency.)
>> Absolutely and in this case it will have to be added both inside the loop and one
>> at the end to cover both the scenarios !
>>
> I'm wondering what are our plans for now?
> Are we going to accept proposed fix just for ARC in 4.4 (and to all stables then)
> or we'll try to come up with more general solution?

I agree with the current solution to add -atomic to for arc700 builds.
Although making that default for arc700 tools will be better but that will not fix
things before next release of tools etc.

But we *do* need to improve generic solution
1. Add atomics detection in perf to add fall back arch stubs
2. ARC needs to add syscall for facilitating atomic r-m-w !

-Vineet