On Tue, Oct 20, 2015 at 08:00:46AM +0000, Vineet Gupta wrote:
> On Monday 19 October 2015 03:22 PM, Peter Zijlstra wrote:
> > On Mon, Oct 19, 2015 at 09:46:35AM +0000, Vineet Gupta wrote:
> >> On ARC we could use the atomic EXchange to implement a user space only
> >> binary
> >> semaphore - these atomic ops will be small duration so it is OK to spin
> >> wait for a
> >> little bit. That's how the old pthread library worked for ARC w/o any
> >> atomic support.
> > That has the obvious problem of lock-holder-preemption and the horrible
> > performance issues that result from that.
> >
> > I think the syscall at least has deterministic behaviour, whereas that
> > userspace spin loop has this abysmal worst case thing.
>
> I don't have issue with adding the syscall per-se. But that comes with it's
> own
> headaches of ABI change - more importantly it requires several things to
> match,
> libc, kernel... It would be easier if change was confined to say perf.
OTOH fixing all those would get you a 'sane' system :-)
> 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.
(also, for ACQUIRE you want the READ_MEMORY_BARRIER() _after_ the
test-and-set control dependency.)
But its your arch..
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