On Wed 16-01-19 10:47:56, Dmitry Vyukov wrote: > On Fri, Jan 11, 2019 at 1:46 PM Tetsuo Handa > <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On 2019/01/11 19:48, Dmitry Vyukov wrote: > > >> How did you arrive to the conclusion that it is harmless? > > >> There is only one relevant standard covering this, which is the C > > >> language standard, and it is very clear on this -- this has Undefined > > >> Behavior, that is the same as, for example, reading/writing random > > >> pointers. > > >> > > >> Check out this on how any race that you might think is benign can be > > >> badly miscompiled and lead to arbitrary program behavior: > > >> https://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong > > > > > > Also there is no other practical definition of data race for automatic > > > data race detectors than: two conflicting non-atomic concurrent > > > accesses. Which this code is. Which means that if we continue writing > > > such code we are not getting data race detection and don't detect > > > thousands of races in kernel code that one may consider more harmful > > > than this one the easy way. And instead will spent large amounts of > > > time to fix some of then the hard way, and leave the rest as just too > > > hard to debug so let the kernel continue crashing from time to time (I > > > believe a portion of currently open syzbot bugs that developers just > > > left as "I don't see how this can happen" are due to such races). > > > > > > > I still cannot catch. Read/write of sizeof(long) bytes at naturally > > aligned address is atomic, isn't it? > > Nobody guarantees this. According to C non-atomic conflicting > reads/writes of sizeof(long) cause undefined behavior of the whole > program. Yes, but to be fair the kernel has always relied on long accesses to be atomic pretty heavily so that it is now de-facto standard for the kernel AFAICT. I understand this makes life for static checkers hard but such is reality. But in this particular case I agree with you that special logic is not really warranted. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
