linux-kernel - Re: [PATCH] rcupdate: move synchronize

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Date:	Tue, 11 Nov 2008 08:55:00 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcupdate: move synchronize_sched() back to rcupdate.c
 V2

Paul E. McKenney wrote:
> On Mon, Nov 10, 2008 at 11:22:15AM +0800, Lai Jiangshan wrote:
>> Paul E. McKenney wrote:
>>> On Thu, Nov 06, 2008 at 02:47:44PM +0800, Lai Jiangshan wrote:
>>>> this fix remove ugly macro, and increase readability for rcupdate codes
>>>>
>>>> changed from v1:
>>>> use HAVE_SPECIAL_RCU_BH/SCHED instead of define duplicate version of
>>>> synchronize_sched().
>>> Hello, Jiangshan!
>>>
>>> I very much like getting rid of the ugly macro.  I of course like the
>>> kernel-doc fixes.  ;-)
>>>
>>> I am not yet convinced of the HAVE_SPECIAL_RCU_BH and
>>> HAVE_SPECIAL_RCU_SCHED pieces.  It is not clear to me that this approach
>>> is simpler than the current approach of simply providing the appropriate
>>> definitions for the symbols in the implementation-specific rcuxxx.h
>>> file.
>>>
>>> Am I missing something?
>>>
>>> 							Thanx, Paul
>>>
>> I think:
>>
>> RCU_BH is not required, we can used RCU instead. so HAVE_SPECIAL_RCU_BH
>> will help for implementation which has not RCU_BH.
>>
>> HAVE_SPECIAL_RCU_SCHED is a little different, RCU and RCU_SCHED are both
>> required for the kernel. But I think, in an implementation,
>> if rcu_read_lock_sched() implies rcu_read_lock(), we may not need implement
>> RCU_SCHED too(sometimes we may implement RCU_SCHED for performance).
>> so HAVE_SPECIAL_RCU_SCHED will help.
> 
> If I understand correctly, this is the "old way":
> 
> ------------------------------------------------------------------------
> 
> rcupdate.h:
> 
> #define rcu_read_lock_bh() __rcu_read_lock_bh()
> #define rcu_read_unlock_bh() __rcu_read_unlock_bh()
> 
> rcupreempt.h:
> 
> #define __rcu_read_lock_bh()    { rcu_read_lock(); local_bh_disable(); }
> #define __rcu_read_unlock_bh()  { local_bh_enable(); rcu_read_unlock(); }
> 
> ------------------------------------------------------------------------
> 
> And then this is the "new way":
> 
> ------------------------------------------------------------------------
> 
> rcupdate.h:
> 
> #ifdef HAVE_SPECIAL_RCU_BH
> #define rcu_read_lock_bh() __rcu_read_lock_bh()
> #define rcu_read_unlock_bh() __rcu_read_unlock_bh()
> #else
> #define __rcu_read_lock_bh()    { rcu_read_lock(); local_bh_disable(); }
> #define __rcu_read_unlock_bh()  { local_bh_enable(); rcu_read_unlock(); }
> #endif /* HAVE_SPECIAL_RCU_BH */
> 
> rcupreempt.h:
> 
> #define HAVE_SPECIAL_RCU_BH
> 
> ------------------------------------------------------------------------
> 
> If we had ten different RCU implementations, then the "new way" would save
> a little bit of code.  But the "old way" is a bit easier to figure out.
> 
> So I am in favor of getting rid of the ugly macro, and also in favor
> of fixing the kerneldoc, but opposed to the HAVE_SPECIAL_RCU_BH and
> HAVE_SPECIAL_RCU_SCHED changes.

I apprehended and agree with you. Thanx.

	Lai.

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