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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>