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

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

On Wed, Oct 14, 2015 at 06:08:16PM +0200, Dmitry Vyukov wrote:
> On Wed, Oct 14, 2015 at 6:01 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Wed, Oct 14, 2015 at 05:50:34PM +0200, 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. 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.
> >
> > So to keep KTSAN happy, any read from some other thread's stack requires
> > READ_ONCE_NOCHECK()?  What if the access is via a locking primitive or
> > read-modify-write atomic operation?
> >
> > This is of some interest in RCU, which implements synchronous grace
> > periods using completions that are allocated on the calling task's stack
> > and manipulated by RCU callbacks that are likely executing elsewhere.
> 
> KTSAN does not have any special logic for stacks. It just generally
> flags pairs of accesses when (1) at least one access is not atomic,
> (2) at least one access is a write and (3) these accesses are not
> synchronized by means of other synchronization.
> There is a bunch of cases when kernel code allocates objects on stack
> and then passes them to other threads, but as far as there is proper
> synchronization it is OK.

OK, so let me see if I understand this.  ;-)

KASAN requires READ_ONCE_NOCHECK() for get_wchan().  KTSAN would be
just as happy with READ_ONCE(), but READ_ONCE_NOCHECK() works for
both.

Did I get it right?

							Thanx, Paul

> For the record, KTSAN is this:
> https://github.com/google/ktsan
> https://github.com/google/ktsan/wiki/Found-Bugs
>