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

[Dmitry Vyukov]

On Wed, Oct 14, 2015 at 6:20 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxxxxx> 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@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. 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?


No, KTSAN also needs READ_ONCE_NOCHECK.
READ_ONCE in get_wchan can lead to a data race report.
Consider:

// the other thead
some_stack_var = ...;

// get_wchan
bp = READ_ONCE(p); // where p happens to point to some_stack_var in
the other thread

This is generally not atomic and not safe. And this is a data race by
all possible definitions.
Only READ_ONCE on reading side is not enough to ensure atomicity, also
all concurrent writes must be done with atomic operations.
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