Re: [PATCH] fs: ratelimit __find_get_block_slow() failure message.

[Dmitry Vyukov <dvyukov@google.com>]

On Fri, Jan 11, 2019 at 1:46 PM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> 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.

> I'm not using increments etc.
> Therefore, in the worst case, some threads see outdated value. But
> outdated values affect only time_in_range() test, which does not cause
> severe problems like crash.
>