linux-kernel - Re: [RFC PATCH 4/5] RCU: Add TASK_RCU

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Message-ID: <4D9256AF.5000308@linux.intel.com>
Date:	Tue, 29 Mar 2011 15:01:19 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Jan Beulich <JBeulich@...ell.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	David Howells <dhowells@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Serge Hallyn <serue@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 4/5] RCU: Add TASK_RCU_OFFSET

On 03/29/2011 02:47 PM, Paul E. McKenney wrote:
> On Tue, Mar 29, 2011 at 02:32:30PM -0700, H. Peter Anvin wrote:
>> On 03/29/2011 02:31 PM, Paul E. McKenney wrote:
>>>>
>>>> I have to say that if we have to use hardcoded offsets in C then we have
>>>> bigger problems.
>>>
>>> In this case, the offsets are mechanically generated from the structure
>>> definitions.
>>>
>>> Or am I missing your point?
>>
>> Yes.  The point is if we have to pull out these kinds of hacks in *C*
>> code, we are doing it wrong.  Not just a little wrong, but completely
>> and totally bonkers wrong.
> 
> OK, maybe we are doing it wrong.
> 
> But in that case, how do you suggest restructuring include/linux/sched.h
> so that struct task_struct can be safely included everywhere
> rcu_read_lock() and friends are invoked?  Or, on the other hand,
> what should we be doing so that we don't need to include task_struct
> everywhere?
> 

Lai's text doesn't give any hint as to the specific nature of the
conflict, which makes it hard to come up with a better alternative
without having to rediscover the problem from first principles.
However, a somewhat logical assumption is that the problem is that
struct task_struct includes struct rcu_head, in which case the easiest
thing to do is almost certainly to move the definition of struct
rcu_head to its own header file, <linux/rcuhead.h>, and include that in
<linux/sched.h>, which should make it possible to include
<linux/sched.h> in <linux/rcupdate.h>.

	-hpa
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