Re: smp_rmb in mm/memory.c in 2.6.10

Andrea Arcangeli
2005-01-14T21:25:33Z

On Fri, Jan 14, 2005 at 08:37:58PM +0000, Hugh Dickins wrote:
> On Thu, 13 Jan 2005, Kanoj Sarcar wrote:
> >
> > Thanks, I think this explains it. IE, if do_no_page()
> > reads truncate_count, and then later goes on to
> > acquire a lock in nopage(), the smp_rmb() is
> > guaranteeing that the read of truncate_count completes
> > before nopage() starts executing.
> >
> > For x86 at least, it seems to me that since the
> > spin_lock (in nopage()) uses a "lock" instruction,
> > that itself guarantees that the truncate_count read is
> > completed, even without the smp_rmb(). (Refer to IA32
> > SDM Vol 3 section 7.2.4 last para page 7-11). Thus for
> > x86, the smp_rmb is superfluous.
>
> You're making me nervous. If you look at 2.6.11-rc1 you'll find
> that I too couldn't see the point of that smp_rmb(), on any architecture,
> and so removed it; while also removing the "atomicity" of truncate_count.
>
> Here was my comment to that patch:
> > Why is mapping->truncate_count atomic? It's incremented inside
> > i_mmap_lock (and i_sem), and the reads don't need it to be atomic.
> >
> > And why smp_rmb() before call to ->nopage? The compiler cannot reorder
> > the initial assignment of sequence after the call to ->nopage, and no
> > cpu (yet!) can read from the future, which is all that matters there.
>
> Now I'm not so convinced by that "no cpu can read from the future".
>
> I don't entirely follow your remarks above, but I do think people
> on this thread have a better grasp of these matters than I have:
> does anyone now think that smp_rmb() needs to be restored?

You could have asked even before breaking mainline ;).

The rmb serializes the read of truncate_count with the read of
inode->i_size. The rmb is definitely required, and I would leave it an
atomic op to be sure gcc doesn't outsmart unmap_mapping_range_list (gcc
can see the internals of unmap_mapping_range_list). I mean just in case.
We must increase that piece of ram before we truncate the ptes and after
we updated the i_size.

Infact it seems to me right now that we miss a smp_wmb() right before
atomic_inc(&mapping->truncate_count): the spin_lock has inclusive
semantics on ia64, and in turn the i_size update could happen after the
atomic_inc without a smp_wmb().

So please backout the buggy changes and add the smp_wmb() to fix this
ia64 altix race.