linux-kernel - Re: [PATCH] rcu: remove SPARSE_RCU

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Message-ID: <20110106055956.GB2506@linux.vnet.ibm.com>
Date:	Wed, 5 Jan 2011 21:59:56 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: remove SPARSE_RCU_POINTER

On Wed, Jan 05, 2011 at 09:30:24AM +0800, Lai Jiangshan wrote:
> On 01/05/2011 05:19 AM, Paul E. McKenney wrote:
> > On Tue, Jan 04, 2011 at 11:36:02AM +0100, Arnd Bergmann wrote:
> >> On Tuesday 04 January 2011 09:43:01 Lai Jiangshan wrote:
> >>> As I known, __rcu annotations do not effect the result compiled kernel.
> >>>
> >>> They work only when we use spare("make C=1" or "make C=2"),
> >>> So we don't need another new switch for it since we have one
> >>> for debugging(use spare or not).
> >>>
> >>> signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> >>
> >> The intention of this option was to avoid introducing an excessive
> >> number of false positives when using sparse.
> >>
> >> We should only make that unconditional if we are reasonably convinced
> >> that all the majority of warnings caused by it should actually
> >> lead to changes in the code.
> > 
> > I agree with Arnd here -- the changes required are extensive in many
> > cases, and a number of subsystems are making decent progress.
> > 
> > 							Thanx, Paul
> 
> I also agree. Most guys like to use direct read to the rcu pointer
> on update side or direct read/write when initializing the rcu pointer.
> This causes a lot of false positives.

And it is good to see some people, most notably Eric Dumazet, making
good use of __rcu!  We will learn from the experiences of the early
adopters, and perhaps be more aggressive pushing __rcu out to the rest
of the community in a year or two, depending on how it goes.

It is quite possible that __rcu will be something that only some of the
maintainers make use of for some time -- and that is OK.

							Thanx, Paul
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