Re: Kernel Concurrency Sanitizer (KCSAN)

Eric Dumazet
2019-10-05T00:58:39Z

On 9/20/19 8:54 AM, Will Deacon wrote:

>
> This one is tricky. What I think we need to avoid is an onslaught of
> patches adding READ_ONCE/WRITE_ONCE without a concrete analysis of the
> code being modified. My worry is that Joe Developer is eager to get their
> first patch into the kernel, so runs this tool and starts spamming
> maintainers with these things to the point that they start ignoring KCSAN
> reports altogether because of the time they take up.
>
> I suppose one thing we could do is to require each new READ_ONCE/WRITE_ONCE
> to have a comment describing the racy access, a bit like we do for memory
> barriers. Another possibility would be to use atomic_t more widely if
> there is genuine concurrency involved.
>

About READ_ONCE() and WRITE_ONCE(), we will probably need

ADD_ONCE(var, value) for arches that can implement the RMW in a single instruction.

WRITE_ONCE(var, var + value) does not look pretty, and increases register pressure.

I had a look at first KCSAN reports, and I can tell that tcp_poll() being lockless
means we need to add hundreds of READ_ONCE(), WRITE_ONCE() and ADD_ONCE() all over the places.

-> Absolute nightmare for future backports to stable branches.