linux-kernel - Re: [patch 0/6] rcu head debugobjects

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Message-ID: <20100329145329.GA2569@linux.vnet.ibm.com>
Date:	Mon, 29 Mar 2010 07:53:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	dipankar@...ibm.com, josh@...htriplett.org, dvhltc@...ibm.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, adobriyan@...il.com
Subject: Re: [patch 0/6] rcu head debugobjects

On Mon, Mar 29, 2010 at 09:39:33AM -0400, Mathieu Desnoyers wrote:
> Hi Paul,
> 
> Thinking about the rcu head init topic, we might be able to drop the
> init_rcu_head() initializer. The idea is the following:
> 
> - We need init_rcu_head_on_stack()/destroy_rcu_head_on_stack().
> - call_rcu() populates the rcu_head and normally does not care about it being
>   pre-initialized.
> - The activation fixup can detect if a non-initialized rcu head is being
>   activated and just perform the fixup without complaining.
> - If we have two call_rcu() in a row in the same GP on the same rcu_head, the
>   activation check will detect it.
> 
> So either we remove all the init_rcu_head(), as was originally proposed, or we
> use one that is a no-op on !DEBUG configs and initialize the object with DEBUG
> configs.
> 
> That removes the dependency on object_is_static().

If I understand correctly, this does sound good.  Here is what I think
you are proposing:

o	call_rcu() and friends only complain if handed an rcu_head
	structure that is still queued awaiting a grace period.
	They don't care otherwise.

o	rcu_do_batch() complains unless the rcu_head structure has
	most recently been enqueued by call_rcu() or one if its friends.

Did I get it right?

							Thanx, Paul
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