linux-kernel - Re: [PATCH 23/32] Generic dynamic per cpu refcounting

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Message-ID: <20130128175304.GX26407@google.com>
Date:	Mon, 28 Jan 2013 09:53:04 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org, linux-fsdevel@...r.kernel.org, zab@...hat.com,
	bcrl@...ck.org, jmoyer@...hat.com, axboe@...nel.dk,
	viro@...iv.linux.org.uk, tytso@....edu,
	Oleg Nesterov <oleg@...hat.com>,
	srivatsa.bhat@...ux.vnet.ibm.com, Christoph Lameter <cl@...ux.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 23/32] Generic dynamic per cpu refcounting

On Fri, Jan 25, 2013 at 04:45:10PM +1030, Rusty Russell wrote:
> Tejun Heo <tj@...nel.org> writes:
> >> 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()).
> >
> > Maybe if we have tryget() which only succeeds if the counter is alive,
> > we can replace moulde refcnt with this?  Rusty?
> 
> Yes, it's similar (hence my previous interest), though module count is a
> bit weird.

I'll try and take a stab at converting it, if I can find time.

> Like Tejun, I'd prefer to see it always alloc up-front, because it
> avoids the _noalloc variant (which is backwards: please hand gfp_t, so
> you don't hide the alloc) and heuristics.

Problem with gfp_t is alloc_percpu() doesn't take it. I don't know why,
but this all goes away with Tejun's idea for allocating from a pool
refilled by workqueue.
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