linux-kernel - Re: [PATCH] Generic percpu refcounting

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Message-ID: <87d2s1rowi.fsf@rustcorp.com.au>
Date:	Wed, 05 Jun 2013 17:05:09 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Kent Overstreet <koverstreet@...gle.com>,
	linux-kernel@...r.kernel.org
Cc:	Kent Overstreet <koverstreet@...gle.com>,
	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>, 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>, Tejun Heo <tj@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul McKenney" <paul.mckenney@...ibm.com>
Subject: Re: [PATCH] Generic percpu refcounting

Kent Overstreet <koverstreet@...gle.com> writes:

> This implements a refcount with similar semantics to
> atomic_get()/atomic_dec_and_test() - but percpu.
>
> It also implements two stage shutdown, as we need it to tear down the
> percpu counts.  Before dropping the initial refcount, you must call
> percpu_ref_kill(); this puts the refcount in "shutting down mode" and
> switches back to a single atomic refcount with the appropriate barriers
> (synchronize_rcu()).
>
> It's also legal to call percpu_ref_kill() multiple times - it only returns
> true once, so callers don't have to reimplement shutdown synchronization.

> +static inline void percpu_ref_get(struct percpu_ref *ref)
> +{
> +	unsigned __percpu *pcpu_count;
> +
> +	preempt_disable();
> +
> +	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> +
> +	if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
> +		__this_cpu_inc(*pcpu_count);
> +	else
> +		atomic_inc(&ref->count);
> +
> +	preempt_enable();
> +}

I think this should be rcu_read_lock(), which is currently equivalent
but theoretically different.

Does your percpu_ref_kill() *really* need to be nonblocking?  (I'd have
to read your other patches which use this to be sure).  Otherwise, just
use synchronize_rcu(), and get rid of the release function...

Cheers,
Rusty.
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