Subject: Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT_RCU
From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
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        peterz@infradead.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com,
        eric.dumazet@gmail.com
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Date: Tue, 17 Aug 2010 09:27:19 -0400
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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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