linux-kernel - Re: [RFC] perf: fix building for ARCv1

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Date:	Tue, 20 Oct 2015 10:45:25 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Noam Camus <noamc@...hip.com>, Andi Kleen <andi@...stfloor.org>,
	"Alexey Brodkin" <Alexey.Brodkin@...opsys.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
	"dvhart@...ux.intel.com" <dvhart@...ux.intel.com>,
	"dsahern@...il.com" <dsahern@...il.com>,
	"acme@...hat.com" <acme@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] perf: fix building for ARCv1

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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