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 !
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