linux-kernel - Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT

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Message-ID: <4C6A580D.5010903@cn.fujitsu.com>
Date:	Tue, 17 Aug 2010 17:36:13 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, 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
Subject: Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT_RCU

On 08/17/2010 06:24 AM, Paul E. McKenney wrote:

>> --(t->rcu_read_lock_nesting)
>>
>> could be split in two distinct operations:
>>
>> read t->rcu_read_lock_nesting
>> decrement t->rcu_read_lock_nesting
>>
>> Note that in order to know the result required to pass the sequence
>> point "&&" (the test), we only need to perform the read, not the
>> decrement. AFAIU, gcc would be in its rights to move the
>> t->rcu_read_lock_nesting update after the volatile access.
> 
> I will run this by some compiler experts.
> 

We can just use "read and decrement statements" instead of "--" to
avoid dependency from compilers.
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