On Mon, May 11, 2015 at 07:24:02AM -0700, Paul E. McKenney wrote: > On Mon, May 11, 2015 at 10:51:17AM +0300, Vladimir Davydov wrote: > > Hi, > > > > I've been arguing with Minchan for a while about whether store-tearing > > is possible while setting page->mapping in __page_set_anon_rmap and > > friends, see > > > > http://thread.gmane.org/gmane.linux.kernel.mm/131949/focus=132132 > > > > This patch is intended to draw attention to this discussion. It fixes a > > race that could happen if store-tearing were possible. The race is as > > follows. > > > > In do_wp_page() we can call page_move_anon_rmap(), which sets > > page->mapping as follows: > > > > anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; > > page->mapping = (struct address_space *) anon_vma; > > > > The page in question may be on an LRU list, because nowhere in > > do_wp_page() we remove it from the list, neither do we take any LRU > > related locks. Although the page is locked, shrink_active_list() can > > still call page_referenced() on it concurrently, because the latter does > > not require an anonymous page to be locked. > > > > If store tearing described in the thread were possible, we could face > > the following race resulting in kernel panic: > > > > CPU0 CPU1 > > ---- ---- > > do_wp_page shrink_active_list > > lock_page page_referenced > > PageAnon->yes, so skip trylock_page > > page_move_anon_rmap > > page->mapping = anon_vma > > rmap_walk > > PageAnon->no > > rmap_walk_file > > BUG > > page->mapping += PAGE_MAPPING_ANON > > > > This patch fixes this race by explicitly forbidding the compiler to > > split page->mapping store in __page_set_anon_rmap() and friends and load > > in PageAnon() with the aid of WRITE/READ_ONCE. > > > > Personally, I don't believe that this can ever happen on any sane > > compiler, because such an "optimization" would only result in two stores > > vs one (note, anon_vma is not a constant), but since I can be mistaken I > > would like to hear from synchronization experts what they think about > > it. > > An example "insane" compiler might notice that the value set cannot be > safely observed without multiple CPUs accessing that variable at the > same time. A paper entitled "No Sane Compiler Would Optimize Atomics" > has some examples: > > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html > > If this paper doesn't scare you, then you didn't read it carefully enough. > And yes, I did give the author a very hard time about the need to suppress > some of these optimizations in order to correctly compile old code, and > will continue to do so. However, a READ_ONCE() would be a most excellent > and very cheap way to future-proof this code, and is highly recommended. Really interesting paper (although scary :-). I think I'm now convinced that a compiler may be really wicked at times. Thank you for sharing the link. Thanks, Vladimir -- 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>