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 b) Add a new, better-named macro to indicate what's going on. Initial bikeshed-able candidates: READ_RO_BITS() READ_IMMUTABLE_BITS() ...etc... thanks, -- John Hubbard NVIDIA
