linux-kernel - Re: [PATCH 04/21] Generic percpu refcounting

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Message-ID: <20130514221513.GA2882@mtj.dyndns.org>
Date:	Tue, 14 May 2013 15:15:13 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-aio@...ck.org, akpm@...ux-foundation.org,
	Zach Brown <zab@...hat.com>, Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jens Axboe <axboe@...nel.dk>,
	Asai Thambi S P <asamymuthupa@...ron.com>,
	Selvan Mani <smani@...ron.com>,
	Sam Bradshaw <sbradshaw@...ron.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 04/21] Generic percpu refcounting

Hello, again, continuing the brain diarrehea,

On Tue, May 14, 2013 at 02:59:45PM -0700, Tejun Heo wrote:
> So, while I do like the simplicity of put() returning %true on the
> final put, I suspect it's more likely to slowing down fast paths due
> to its interface compared to having separate ->release function
> combined with void put().  Any ideas?

Maybe we can structure put in a way that's difficult to get wrong for
the compiler?

	bool put()
	{
		preempt_disable();
		if (likely(not killed yet)) {
			this_cpu_dec();
			preempt_enable();
			return false;
		}
		return put_slowpath();
	}

This doesn't solve the caller not inlining hot path but well I suppose
we can consider that the caller's problem.  The above at least
wouldn't introduce an unnecessary branch on its own.

Thanks.

-- 
tejun
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