[RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed

[Peter Zijlstra]

On Fri, Nov 02, 2018 at 07:19:15PM +0300, Andrey Ryabinin wrote:

> UBSAN warns about signed overflows despite -fno-strict-overflow if gcc
> version is < 8.  I have learned recently that UBSAN in GCC 8 ignores
> signed overflows if -fno-strict-overflow of fwrapv is used.

Ah, good.

> We can always just drop -fsanitize=signed-integer-overflow if it considered too noisy.

I think that is the most consistent beahviour. signed overflow is not UB
in the kernel.

> Although it did catch some real bugs.

If we want an over/under-flow checker, then that should be a separate
plugin and not specific to signed or unsigned.