Re: pagefault_disable (was Re: [patch 6/6] mm: fix pagecache writedeadlocks)

[Nick Piggin]

Peter Zijlstra wrote:
> On Tue, 2006-10-17 at 01:24 +1000, Nick Piggin wrote:

> > Also, the rest of the kernel tree (mainly uaccess and futexes) should be
> > converted ;)
>
>
> Yeah, lotsa places to touch.
>
>
> > > Index: linux-2.6/mm/filemap.h
> > > ===================================================================
> > > --- linux-2.6.orig/mm/filemap.h	2006-10-14 20:20:20.000000000 +0200
> > > +++ linux-2.6/mm/filemap.h	2006-10-15 17:17:45.000000000 +0200
> > > @@ -21,6 +21,22 @@ __filemap_copy_from_user_iovec_inatomic(
> > > 					size_t bytes);
> > >
> > > /*
> > > + * By increasing the preempt_count we make sure the arch preempt
> > > + * handler bails out early, before taking any locks, so that the copy
> > > + * operation gets terminated early.
> > > + */
> > > +pagefault_static inline void disable(void)
> > > +{
> > > +	inc_preempt_count();
>
>
> I think we also need a barrier(); here. We need to make sure the preempt
> count is written to memory before we hit the fault handler.

It will come from this thread, but I guess the fault is not an event the
compiler can forsee, so indeed it might optimise this into the wrong place.
Perhaps not with any copy*user implementation we have, but at least in
theory...

> > > +pagefault_static inline void enable(void)
> > > +{
> > > +	dec_preempt_count();
> > > +	preempt_check_resched();
> > > +}

You'll want barriers before and after the dec_preempt_count, for similar
reasons.

--
SUSE Labs, Novell Inc.
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