On 2/6/20 4:18 PM, Qian Cai wrote: >> On Feb 6, 2020, at 6:34 PM, John Hubbard <jhubbard@xxxxxxxxxx> wrote: >> On 2/6/20 7:23 AM, Qian Cai wrote: >>>> On Feb 6, 2020, at 9:55 AM, Jan Kara <jack@xxxxxxx> wrote: >>>> I don't think the problem is real. The question is how to make KCSAN happy >>>> in a way that doesn't silence other possibly useful things it can find and >>>> also which makes it most obvious to the reader what's going on... IMHO >>>> using READ_ONCE() fulfills these targets nicely - it is free >>>> performance-wise in this case, it silences the checker without impacting >>>> other races on page->flags, its kind of obvious we don't want the load torn >>>> in this case so it makes sense to the reader (although a comment may be >>>> nice). >>> >>> Actually, use the data_race() macro there fulfilling the same purpose too, i.e, silence the splat here but still keep searching for other races. >>> >> >> Yes, but both READ_ONCE() and data_race() would be saying untrue things about this code, >> and that somewhat offends my sense of perfection... :) >> >> * READ_ONCE(): this field need not be restricted to being read only once, so the >> name is immediately wrong. We're using side effects of READ_ONCE(). >> >> * data_race(): there is no race on the N bits worth of page zone number data. There >> is only a perceived race, due to tools that look at word-level granularity. >> >> I'd propose one or both of the following: >> >> a) Hope that Marco (I've fixed the typo in his name. --jh) has an idea to enhance KCSAN so as to support this model of >> access, and/or > > A similar thing was brought up before, i.e., anything compared to zero is immune to load-tearing > issues, but it is rather difficult to implement it in the compiler, so it was settled to use data_race(), > > https://lore.kernel.org/lkml/CANpmjNN8J1oWtLPHTgCwbbtTuU_Js-8HD=cozW5cYkm8h-GTBg@xxxxxxxxxxxxxx/#r > Thanks for that link to the previous discussion, good context. >> >> b) Add a new, better-named macro to indicate what's going on. Initial bikeshed-able >> candidates: >> >> READ_RO_BITS() >> READ_IMMUTABLE_BITS() >> ...etc... >> > > Actually, Linus might hate those kinds of complication rather than a simple data_race() macro, > > https://lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@xxxxxxxxxxxxxx/ > Another good link. However, my macros above haven't been proposed yet, and I'm perfectly comfortable proposing something that Linus *might* (or might not!) hate. No point in guessing about it, IMHO. If you want, I'll be happy to put on my flame suit and post a patchset proposing READ_IMMUTABLE_BITS() (or a better-named thing, if someone has another name idea). :) thanks, -- John Hubbard NVIDIA
