On Tue, May 24, 2022 at 09:37:28AM -0700, Paul E. McKenney wrote: > On Tue, May 24, 2022 at 12:48:31PM -0300, Jason Gunthorpe wrote: > > On Tue, May 24, 2022 at 08:43:27AM -0700, Minchan Kim wrote: > > > On Tue, May 24, 2022 at 11:19:37AM -0300, Jason Gunthorpe wrote: > > > > On Mon, May 23, 2022 at 10:16:58PM -0700, Minchan Kim wrote: > > > > > On Mon, May 23, 2022 at 07:55:25PM -0700, John Hubbard wrote: > > > > > > On 5/23/22 09:33, Minchan Kim wrote: > > > > > > ... > > > > > > > > So then: > > > > > > > > > > > > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > > > > > > > index 0e42038382c1..b404f87e2682 100644 > > > > > > > > +++ b/mm/page_alloc.c > > > > > > > > @@ -482,7 +482,12 @@ unsigned long __get_pfnblock_flags_mask(const struct page *page, > > > > > > > > word_bitidx = bitidx / BITS_PER_LONG; > > > > > > > > bitidx &= (BITS_PER_LONG-1); > > > > > > > > > > > > > > > > - word = bitmap[word_bitidx]; > > > > > > > > + /* > > > > > > > > + * This races, without locks, with set_pageblock_migratetype(). Ensure > > > > > > > set_pfnblock_flags_mask would be better? > > > > > > > > + * a consistent (non-tearing) read of the memory array, so that results, > > > > > > > > > > > > > > Thanks for proceeding and suggestion, John. > > > > > > > > > > > > > > IIUC, the load tearing wouldn't be an issue since [1] fixed the issue. > > > > > > > > > > > > Did it? [1] fixed something, but I'm not sure we can claim that that > > > > > > code is now safe against tearing in all possible cases, especially given > > > > > > the recent discussion here. Specifically, having this code do a read, > > > > > > then follow that up with calculations, seems correct. Anything else is > > > > > > > > > > The load tearing you are trying to explain in the comment would be > > > > > solved by [1] since the bits will always align on a word and accessing > > > > > word size based on word aligned address is always atomic so there is > > > > > no load tearing problem IIUC. > > > > > > > > That is not technically true. It is exactly the sort of thing > > > > READ_ONCE is intended to guard against. > > > > > > Oh, does word access based on the aligned address still happen > > > load tearing? > > > > > > I just referred to > > > https://elixir.bootlin.com/linux/latest/source/Documentation/memory-barriers.txt#L1759 > > > > I read that as saying load tearing is technically allowed but doesn't > > happen in gcc, and so must use the _ONCE macros. > > This is in fact the intent, except... > > And as that passage goes on to state, there really are compilers (such > as GCC) that tear stores of constants to machine aligned/sized locations. > > In short, use of the _ONCE() macros can save you a lot of pain. Thanks for the correction, Jason and Paul > > > > I didn't say it doesn't refetch the value without the READ_ONCE. > > > > > > What I am saying is READ_ONCE(bitmap_word_bitidx] prevents "refetching" > > > issue rather than "tearing" issue in specific __get_pfnblock_flags_mask > > > context because I though there is no load-tearing issue there since > > > bitmap is word-aligned/accessed. No? > > > > It does both. AFAIK our memory model has no guarentees on what naked C > > statements will do. Tearing, multi-load, etc - it is all technically > > permitted. Use the proper accessors. Seems like there was some misunderstanding here. I didn't mean not to use READ_ONCE for the bitmap but wanted to have more concrete comment. Since you guys corrected "even though word-alinged access could be wrong without READ_ONCE", I would keep the comment John suggested. > > I am with Jason on this one. > > In fact, I believe that any naked C-language access to mutable shared > variables should have a comment stating why the compiler cannot mangle > that access. Agreed. Thanks!
