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

[Johannes Weiner <hannes@cmpxchg.org>]

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.

The definition of ACCESS_ONCE() relies on gcc's current
implementation, the users of ACCESS_ONCE() only rely on ACCESS_ONCE()
being defined.

Should it ever break you have to either fix it at the implementation
level or remove/replace the abstraction in its entirety, how does the
individual callsite matter in this case?

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