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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Oct 14, 2015 at 06:29:59PM +0200, Dmitry Vyukov wrote:
> On Wed, Oct 14, 2015 at 6:19 PM, Andrey Ryabinin
> <aryabinin@virtuozzo.com> wrote:
> >
> >
> > 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?
> 
> 
> 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.

Where possible, it would be better to make the normal write instead
be WRITE_ONCE().  That might well not be possible here, but let's not
be too aggressive about silencing KTSAN.

							Thanx, Paul