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Date: Fri, 16 Apr 2010 18:18:59 +1000
From: Nick Piggin <npiggin@suse.de>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: paulmck@linux.vnet.ibm.com, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Andrea Arcangeli <aarcange@redhat.com>, Avi Kivity <avi@redhat.com>,
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       David Miller <davem@davemloft.net>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>, Mel Gorman <mel@csn.ul.ie>
Subject: Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast()
 implementation
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On Fri, Apr 16, 2010 at 04:51:34PM +1000, Benjamin Herrenschmidt 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().
> 
> Ok, so I'm a bit of a RCU newbie as you may know :-) Right now, we use
> neither, we use call_rcu and we free the pages from the callback.

BTW. you currently have an interesting page table freeing path where
you usually free by RCU, but (occasionally) free by IPI. This means
you need to disable both RCU and interrupts to walk page tables.

If you change it to always use RCU, then you wouldn't need to disable
interrupts. Whether this actually matters anywhere in your mm code, I
don't know (it's probably not terribly important for gup_fast). But
rcu disable is always preferable for latency and performance.