Messages in this thread | | | Date | Mon, 12 Apr 2010 20:43:11 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation |
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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); > > > > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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