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

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Date:	Mon, 19 Oct 2015 11:51:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
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 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.
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