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

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Date:	Mon, 19 Oct 2015 09:46:35 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Aabid Rushdi <Aabid.Rushdi@...opsys.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>
Subject: Re: [RFC] perf: fix building for ARCv1

On Monday 19 October 2015 03:05 PM, Peter Zijlstra wrote:
> On Mon, Oct 19, 2015 at 09:28:43AM +0000, Vineet Gupta wrote:
>> > On Monday 19 October 2015 11:20 AM, Andi Kleen wrote:
>>> > > Vineet Gupta <Vineet.Gupta1@...opsys.com> writes:
>>>> > >> But this user space - so IMHO UP/SMP doesn't matter and we can't simulate them in
>>>> > >> C just by itself.
>>> > > It matters when you access the perf ring buffer which is updated by kernel.
>> > 
>> > That's part of the problem. The issue is with atomic_* APIs proliferation in perf
>> > user space code which assumes native atomix r-m-w support which is not always
>> > true. So I think we still need a feature detection mechanism and if absent leave
>> > the ball in arch court by calling arch_atomic_* which can use creative or half
>> > working measures so perf will work to some extent atleast and not bomb outright.
>> > 
>> > Also can u please elaborate a bit on "simulate them in C" - u mean just simple
>> > unprotected LD, OP, ST or do u fancy usage of futex etc?
> Doesn't ARMv5 have a cmpxchg syscall to deal with this? It does an
> IRQ-disabled load-op-store sequence.

Yeah I remember seeing some syscall like that in ARM.

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.

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