Re: [PATCH] mm: prevent mmap_cache race in find_vma()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Apr 02, 2013 at 04:55:45PM -0700, David Rientjes wrote:
> On Tue, 2 Apr 2013, Hugh Dickins wrote:
> 
> > > > find_vma() can be called by multiple threads with read lock
> > > > held on mm->mmap_sem and any of them can update mm->mmap_cache.
> > > > Prevent compiler from re-fetching mm->mmap_cache, because other
> > > > readers could update it in the meantime:
> > > 
> > > FWIW, ACCESS_ONCE() does not guarantee that the compiler will not refetch 
> > > mm->mmap_cache whatsoever; there is nothing that prevents this either in 
> > > the C standard.  You'll be relying solely on gcc's implementation of how 
> > > it dereferences volatile-qualified pointers.
> > 
> > Jan is using ACCESS_ONCE() as it should be used, for its intended
> > purpose.  If the kernel's implementation of ACCESS_ONCE() is deficient,
> > then we should fix that, not discourage its use.
> > 
> 
> My comment is about the changelog, quoted above, saying "prevent compiler 
> from re-fetching mm->mmap_cache..."  ACCESS_ONCE(), as implemented, does 
> not prevent the compiler from re-fetching anything.  It is entirely 
> plausible that in gcc's current implementation that this guarantee is 
> made, but it is not prevented by the language standard and I think the 
> changelog should be reworded for anybody who reads it in the future.  
> There is a dependency here on gcc's implementation, it's a meaningful 
> distinction.
> 
> I never discouraged its use since for gcc's current implementation it 
> appears to work as desired and without gcc extensions there is no way to 
> make such a guarantee by the standard.  In fact, I acked a patch from Eric 
> Dumazet that fixes a NULL pointer dereference by using ACCESS_ONCE() with 
> gcc in slub.

This LWN comment from user "nix" is helpful here:

https://lwn.net/Articles/509731/

In particular:

	... volatile's meaning as 'minimize optimizations applied to
	things manipulating anything of volatile type, do not duplicate,
	elide, move, fold, spindle or mutilate' is of long standing.

So although I agree that the standard does not say as much as one might
like about volatile, ACCESS_ONCE()'s use of volatile should be expected
to work in a wide range of C compilers.  ACCESS_ONCE()'s use of typeof()
might not be quite so generally applicable, but a fair range of C
compilers do seem to support typeof() as well as ACCESS_ONCE()'s use
of volatile.

							Thanx, Paul

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