On Tue, Nov 16, 2021 at 4:04 AM Marco Elver <elver@xxxxxxxxxx> wrote: > > On Mon, Nov 15, 2021 at 11:59AM -0800, Shakeel Butt wrote: > > On Mon, Nov 15, 2021 at 10:55 AM Mina Almasry <almasrymina@xxxxxxxxxx> wrote: > [...] > > > Sorry I'm still a bit confused. READ_ONCE/WRITE_ONCE isn't documented > > > to provide atomicity to the write or read, just prevents the compiler > > > from re-ordering them. Is there something I'm missing, or is the > > > suggestion to add READ_ONCE/WRITE_ONCE simply to supress the KCSAN > > > warnings? > > It's actually the opposite: READ_ONCE/WRITE_ONCE provide very little > ordering (modulo dependencies) guarantees, which includes ordering by > compiler, but are supposed to provide atomicity (when used with properly > aligned types up to word size [1]; see __READ_ONCE for non-atomic > variant). > > Some more background... > > The warnings that KCSAN tells you about are "data races", which occur > when you have conflicting concurrent accesses, one of which is "plain" > and at least one write. I think [2] provides a reasonable summary of > data races and why we should care. > > For Linux, our own memory model (LKMM) documents this [3], and says that > as long as concurrent operations are marked (non-plain; e.g. *ONCE), > there won't be any data races. > > There are multiple reasons why data races are undesirable, one of which > is to avoid bad compiler transformations [4], because compilers are > oblivious to concurrency otherwise. > > Why do marked operations avoid data races and prevent miscompiles? > Among other things, because they should be executed atomically. If they > weren't a lot of code would be buggy (there had been cases where the old > READ_ONCE could be used on data larger than word size, which certainly > weren't atomic, but this is no longer possible). > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/asm-generic/rwonce.h#n35 > [2] https://lwn.net/Articles/816850/#Why%20should%20we%20care%20about%20data%20races? > [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt#n1920 > [4] https://lwn.net/Articles/793253/ > > Some rules of thumb when to use which marking: > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt > > In an ideal world, we'd have all intentionally concurrent accesses > marked. As-is, KCSAN will find: > > A. Data race, where failure due to current compilers is unlikely > (supposedly "benign"); merely marking the accesses appropriately is > sufficient. Finding a crash for these will require a miscompilation, > but otherwise look "benign" at the C-language level. > > B. Race-condition bugs where the bug manifests as a data race, too -- > simply marking things doesn't fix the problem. These are the types of > bugs where a data race would point out a more severe issue. > > Right now we have way too much of type (A), which means looking for (B) > requires patience. > > > +Paul & Marco > > > > Let's ask the experts. > > > > We have a "unsigned long usage" variable that is updated within a lock > > (hugetlb_lock) but is read without the lock. > > > > Q1) I think KCSAN will complain about it and READ_ONCE() in the > > unlocked read path should be good enough to silent KCSAN. So, the > > question is should we still use WRITE_ONCE() as well for usage within > > hugetlb_lock? > > KCSAN's default config will forgive the lack of WRITE_ONCE(). > Technically it's still a data race (which KCSAN can find with a config > change), but can be forgiven because compilers are less likely to cause > trouble for writes (background: https://lwn.net/Articles/816854/ bit > about "Unmarked writes (aligned and up to word size)..."). > > I would mark both if feasible, as it clearly documents the fact the > write can be read concurrently. > > > Q2) Second question is more about 64 bit archs breaking a 64 bit write > > into two 32 bit writes. Is this a real issue? If yes, then the > > combination of READ_ONCE()/WRITE_ONCE() are good enough for the given > > use-case? > > Per above, probably unlikely, but allowed. WRITE_ONCE should prevent it, > and at least relieve you to not worry about it (and shift the burden to > WRITE_ONCE's implementation). > Thank you very much for the detailed response. I can add READ_ONCE() at the no-lock read site, that is no issue. However, for the writes that happen while holding the lock, the write is like so: + h_cg->nodeinfo[page_to_nid(page)]->usage[idx] += nr_pages; And like so: + h_cg->nodeinfo[page_to_nid(page)]->usage[idx] -= nr_pages; I.e. they are increments/decrements. Sorry if I missed it but I can't find an INC_ONCE(), and it seems wrong to me to do something like: + WRITE_ONCE(h_cg->nodeinfo[page_to_nid(page)]->usage[idx], + h_cg->nodeinfo[page_to_nid(page)] + nr_pages); I know we're holding a lock anyway so there is no race, but to the casual reader this looks wrong as there is a race between the fetch of the value and the WRITE_ONCE(). What to do here? Seems to me the most reasonable thing to do is just READ_ONCE() and leave the write plain? > Thanks, > -- Marco
