Re: [PATCH v4 1/2] compiler, atomics: Provide READ_ONCE_NOKSAN()

[Peter Zijlstra]

On Fri, Oct 16, 2015 at 12:44:53PM +0300, Andrey Ryabinin wrote:
> Some code may perform racy by design memory reads. This could be
> harmless, yet such code may produce KASAN warnings.
> 
> To hide such accesses from KASAN this patch introduces
> READ_ONCE_NOKSAN() macro. KASAN will not check the memory
> accessed by READ_ONCE_NOKSAN(). The KernelThreadSanitizer (KTSAN)
> is going to ignore it as well.
> 
> This patch creates __read_once_size_noksan() a clone of
> __read_once_size(). The only difference between them is
> 'no_sanitized_address' attribute appended to '*_nokasan' function.
> This attribute tells the compiler that instrumentation of memory
> accesses should not be applied to that function. We declare it as
> static '__maybe_unsed' because GCC is not capable to inline such
> function: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
> 
> With KASAN=n READ_ONCE_NOKSAN() is just a clone of READ_ONCE().

Would we need a similar annotation for things like
mutex_spin_on_owner()'s dereference of owner, or is that considered safe
by KASAN?

(its not actually safe; as I remember we have a problem with using
rcu_read_lock for tasks like that)
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