> 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 Marcus 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 > > 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/
