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

[Paul E. McKenney]

On Wed, Oct 14, 2015 at 07:23:02PM +0200, Dmitry Vyukov wrote:
> 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 :)

But a non-data-race out of bounds would presumably trigger KASAN but not
KTSAN.

> > 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.

Good!

							Thanx, Paul

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