Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation

Paul E. McKenney
2010-04-13T03:43:15Z

On Tue, Apr 13, 2010 at 11:05:31AM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2010-04-08 at 21:17 +0200, Peter Zijlstra wrote:
> > plain text document attachment (powerpc-gup_fast-rcu.patch)
> > The powerpc page table freeing relies on the fact that IRQs hold off
> > an RCU grace period, this is currently true for all existing RCU
> > implementations but is not an assumption Paul wants to support.
> >
> > Therefore, also take the RCU read lock along with disabling IRQs to
> > ensure the RCU grace period does at least cover these lookups.
>
> There's a few other places that need a similar fix then. The hash page
> code for example. All the C cases should end up calling the
> find_linux_pte() helper afaik, so we should be able to stick the lock in
> there (and the hugetlbfs variant, find_linux_pte_or_hugepte()).
>
> However, we also have cases of tight asm code walking the page tables,
> such as the tlb miss handler on embedded processors. I don't see how I
> could do that there. IE. I only have a handful of registers to play
> with, no stack, etc...
>
> 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().

Thanx, Paul

> Cheers,
> Ben.
>
> > Requested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: Nick Piggin <npiggin@suse.de>
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > ---
> > arch/powerpc/mm/gup.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > Index: linux-2.6/arch/powerpc/mm/gup.c
> > ===================================================================
> > --- linux-2.6.orig/arch/powerpc/mm/gup.c
> > +++ linux-2.6/arch/powerpc/mm/gup.c
> > @@ -142,6 +142,7 @@ int get_user_pages_fast(unsigned long st
> > * So long as we atomically load page table pointers versus teardown,
> > * we can follow the address down to the the page and take a ref on it.
> > */
> > + rcu_read_lock();
> > local_irq_disable();
> >
> > pgdp = pgd_offset(mm, addr);
> > @@ -162,6 +163,7 @@ int get_user_pages_fast(unsigned long st
> > } while (pgdp++, addr = next, addr != end);
> >
> > local_irq_enable();
> > + rcu_read_unlock();
> >
> > VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
> > return nr;
> > @@ -171,6 +173,7 @@ int get_user_pages_fast(unsigned long st
> >
> > slow:
> > local_irq_enable();
> > + rcu_read_unlock();
> > slow_irqon:
> > pr_devel(" slow path ! nr = %d\n", nr);
> >
> >
>
>
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