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

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Message-ID: <1282051639.3268.1335.camel@gandalf.stny.rr.com>
Date:	Tue, 17 Aug 2010 09:27:19 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.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 Mon, 2010-08-16 at 18:07 -0400, Mathieu Desnoyers wrote:

> > Moving this down past the check of t->rcu_read_lock_special (which is
> > now covered by ACCESS_ONCE()) would violate the C standard, as it would
> > be equivalent to moving a volatile up past a sequence point.
> 
> Hrm, I'm not quite convinced yet. I am not concerned about gcc moving
> the volatile access prior to the sequence point (as you say, this is
> forbidden by the C standard), but rather that:
> 
> --(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.
> 

If we are this concerned, what about just doing:

	--t->rcu_read_lock_nesting;
        if (ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
             unlikely((ACCESS_ONCE(t->rcu_read_unlock_special)))

-- Steve


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