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

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Message-ID: <20100817143514.GA2471@linux.vnet.ibm.com>
Date:	Tue, 17 Aug 2010 07:35:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.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 Tue, Aug 17, 2010 at 05:36:13PM +0800, Lai Jiangshan wrote:
> 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.

You mean something like local_add_return()?  This turns into
atomic_add_return() on many platforms, including x86 as it turns out,
so I am reluctant to use it.

If you had something else in mind, please don't keep it a secret!

							Thanx, Paul
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