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Subject: Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast()
 implementation
From: Peter Zijlstra <peterz@infradead.org>
To: paulmck@linux.vnet.ibm.com
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Date: Fri, 16 Apr 2010 15:51:21 +0200
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On Thu, 2010-04-15 at 07:28 -0700, Paul E. McKenney wrote:
> On Wed, Apr 14, 2010 at 03:51:50PM +0200, Peter Zijlstra wrote:
> > On Mon, 2010-04-12 at 20:43 -0700, Paul E. McKenney wrote:
> > > > So we might have to support the interrupt assumption, at least in some
> > > > form, with those guys...
> > > 
> > > One way to make the interrupt assumption official is to use
> > > synchronize_sched() rather than synchronize_rcu().
> > 
> > Well, call_rcu_sched() then, because the current usage is to use
> > call_rcu() to free the page directories.
> > 
> > Paul, here is a call_rcu_sched() available in kernel/rcutree.c, but am I
> > right in reading that code that that would not be available for
> > preemptible RCU?
> 
> Both call_rcu_sched() and call_rcu() are always there for you.  ;-)
> 
> o	If CONFIG_TREE_RCU (or CONFIG_TINY_RCU), they both have the same
> 	implementation.
> 
> o	If CONFIG_TREE_PREEMPT_RCU, call_rcu_sched() is preemptible and
> 	call_rcu() is not.

(The reverse I suspect?)

> Of course, with call_rcu_sched(), the corresponding RCU read-side critical
> sections are non-preemptible.  Therefore, in CONFIG_PREEMPT_RT, these
> read-side critical sections must use raw spinlocks.

OK, so if we fully remove CONFIG_TREE_PREEMPT_RCU (defaulting to y),
rename all the {call_rcu, rcu_read_lock, rcu_read_unlock,
synchronize_rcu} functions to {*}_preempt and then add a new
CONFIG_PREEMPT_RCU that simply maps {*} to either {*}_sched or
{*}_preempt, we've basically got what I've been asking for for a while,
no?

> Can the code in question accommodate these restrictions?

Yes, that should do just fine I think.