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Date: Fri, 16 Apr 2010 07:32:02 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Andrea Arcangeli <aarcange@redhat.com>, Avi Kivity <avi@redhat.com>,
       Thomas Gleixner <tglx@linutronix.de>, Rik van Riel <riel@redhat.com>,
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       Linus Torvalds <torvalds@linux-foundation.org>,
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       David Miller <davem@davemloft.net>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>, Mel Gorman <mel@csn.ul.ie>,
       Nick Piggin <npiggin@suse.de>
Subject: Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast()
 implementation
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On Fri, Apr 16, 2010 at 04:23:39PM +0200, Peter Zijlstra wrote:
> On Fri, 2010-04-16 at 07:17 -0700, Paul E. McKenney wrote:
> 
> > > > 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?
> > 
> > What would rcu_read_lock_preempt() do in a !CONFIG_PREEMPT kernel?
> 
> Same as for a preempt one, since you'd have to be able to schedule()
> while holding it to be able to do things like mutex_lock().

So what you really want is something like rcu_read_lock_sleep() rather
than rcu_read_lock_preempt(), right?  The point is that you want to do
more than merely preempt, given that it is legal to do general blocking
while holding a mutex, correct?

							Thanx, Paul