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

[Dmitry Vyukov]

On Wed, Oct 14, 2015 at 7:04 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.
>
> OK.  However, this is specific to get_wchan()'s out-of-bounds stack, right?

Yes... and no.
This is specific to racy accesses, not necessary to out-of-bounds
stack. If you have a data race in-bounds, it is also not OK :)


> If I have multiple tasks accessing some other task's on-stack variable,
> then as long as all other potentially concurrent accesses use atomic
> operations, READ_ONCE() suffices.  Or am I still missing something here?

This is correct.
Generally, the idea is that KTSAN flags what you think is a bug, and
does not flag what you think is not a bug. If you have all proper
synchronization in place, then KTSAN should not flag that; whether the
object is on stack or not is irrelevant.
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