From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Herrenschmidt Subject: Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation Date: Tue, 13 Apr 2010 11:05:31 +1000 Message-ID: <1271120731.13059.6.camel@pasglop> References: <20100408191737.296180458@chello.nl> <20100408192722.643778654@chello.nl> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: Received: from gate.crashing.org ([63.228.1.57]:54824 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754407Ab0DMBHr (ORCPT ); Mon, 12 Apr 2010 21:07:47 -0400 In-Reply-To: <20100408192722.643778654@chello.nl> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Andrea Arcangeli , Avi Kivity , Thomas Gleixner , Rik van Riel , Ingo Molnar , akpm@linux-foundation.org, Linus Torvalds , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, David Miller , Hugh Dickins , Mel Gorman , Nick Piggin , "Paul E. McKenney" 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 > Signed-off-by: Peter Zijlstra > Cc: Nick Piggin > Cc: Benjamin Herrenschmidt > --- > 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); > >