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Date: Tue, 20 Oct 2015 12:11:33 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: Noam Camus <noamc@ezchip.com>, Andi Kleen <andi@firstfloor.org>,
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        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC] perf: fix building for ARCv1
Message-ID: <20151020101133.GA17308@twins.programming.kicks-ass.net>
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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..