Re: [tip:locking/urgent] compiler, atomics: Provide READ_ONCE_NOCHECK ()

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 10/14/2015 06:50 PM, Dmitry Vyukov wrote:
> On Wed, Oct 14, 2015 at 5:45 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
>> On Wed, Oct 14, 2015 at 08:28:43AM -0700, tip-bot for Andrey Ryabinin wrote:
>>> Commit-ID:  4115ffdf4d6f8986a7abe1dd522c163f599bc0e6
>>> Gitweb:     http://git.kernel.org/tip/4115ffdf4d6f8986a7abe1dd522c163f599bc0e6
>>> Author:     Andrey Ryabinin <aryabinin@virtuozzo.com>
>>> AuthorDate: Tue, 13 Oct 2015 18:28:07 +0300
>>> Committer:  Ingo Molnar <mingo@kernel.org>
>>> CommitDate: Wed, 14 Oct 2015 16:44:06 +0200
>>>
>>> compiler, atomics: Provide READ_ONCE_NOCHECK()
>>>
>>> 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_NOCHECK() macro. KASAN will not check the memory
>>> accessed by READ_ONCE_NOCHECK().
>>>
>>> This patch creates __read_once_size_nocheck() a clone of
>>> __read_once_size_check() (renamed __read_once_size()).
>>> The only difference between them is 'no_sanitized_address'
>>> attribute appended to '*_nocheck' 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_NOCHECK() is just a clone of READ_ONCE().
>>
>> So I add READ_ONCE_NOCHECK() for accesses for which the compiler cannot
>> prove safe address for KASAN's benefit, but READ_ONCE() suffices for
>> the data-race-detection logic in KTSAN, correct?
> 
> KTSAN also needs READ_ONCE_NOCHECK() here.

Does it? What's the difference between READ_ONCE_NOCHECK() and READ_ONCE() with KTSAN=y?
AFAIK READ_ONCE() is sufficient to hide race from KTSAN. It doesn't *require* READ_ONCE_NOCHECK(), right?

> KTSAN will flag races
> between get_wchan() and the thread accesses to own stack even more
> aggressively than KASAN, because KTSAN won't like get_wchan() accesses
> even to non-poisoned areas of other thread stack.
>