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

Benjamin Herrenschmidt
2010-04-13T01:07:47Z

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...

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);
>
>