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

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Message-ID: <87libhpz4h.fsf@rustcorp.com.au>
Date:	Fri, 25 Jan 2013 16:45:10 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Tejun Heo <tj@...nel.org>, Kent Overstreet <koverstreet@...gle.com>
Cc:	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

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.

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.

Cheers,
Rusty.
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