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

[Dmitry Vyukov]

On Wed, Oct 14, 2015 at 6:19 PM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> wrote:
>
>
> On 10/14/2015 06:50 PM, Dmitry Vyukov wrote:
>> On Wed, Oct 14, 2015 at 5:45 PM, Paul E. McKenney
>> <paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>>>> AuthorDate: Tue, 13 Oct 2015 18:28:07 +0300
>>>> Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
>>>> 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?


For not there is no difference, because you just added
READ_ONCE_NOCHECK and we have not yet supported it.
But my plan is to completely ignore accessed from READ_ONCE_NOCHECK in
KTSAN so that they never lead to race reports.

READ_ONCE in get_wchan still can lead to a data race report, because
it is READ_ONCE in get_wchan versus a normal write to stack in the
other thread. That is not atomic and not generally safe.
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