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

[David Rientjes <rientjes@google.com>]

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.

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