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

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Message-ID: <20130514215945.GA2334@mtj.dyndns.org>
Date:	Tue, 14 May 2013 14:59:45 -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

A couple more things.

On Mon, May 13, 2013 at 06:18:41PM -0700, Kent Overstreet wrote:
...
> +/**
> + * percpu_ref_put - decrement a dynamic percpu refcount
> + *
> + * Returns true if the result is 0, otherwise false; only checks for the ref
> + * hitting 0 after percpu_ref_kill() has been called. Analagous to
> + * atomic_dec_and_test().
> + */
> +static inline int percpu_ref_put(struct percpu_ref *ref)

bool?

> +{
> +	unsigned __percpu *pcpu_count;
> +	int ret = 0;
> +
> +	preempt_disable();
> +
> +	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> +
> +	if (pcpu_count)

We probably want likely() here.

> +		__this_cpu_dec(*pcpu_count);
> +	else
> +		ret = atomic_dec_and_test(&ref->count);
> +
> +	preempt_enable();
> +
> +	return ret;

With likely() added, I think the compiler should be able to recognize
that the branch on pcpu_count should exclude later branch in the
caller to test for the final put in most cases but I'm a bit worried
whether that would always be the case and wonder whether ->release
based interface would be better.  Another concern is that the above
interface is likely to encourage its users to put the release
implementation in the same function.  e.g.

	void my_put(my_obj)
	{
		if (!percpu_ref_put(&my_obj->ref))
			return;
		destroy my_obj;
		free my_obj;
	}

Which in turn is likely to nudge the developer or compiler towards not
inlining the fast path.

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?

Thanks.

-- 
tejun
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