Re: [PATCH 2/3] kcsan: Introduce ASSERT_EXCLUSIVE_* macros

[Marco Elver]

On Wed, 5 Feb 2020 at 22:33, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Wed, Feb 05, 2020 at 09:43:32PM +0100, Marco Elver wrote:
> > Introduces ASSERT_EXCLUSIVE_WRITER and ASSERT_EXCLUSIVE_ACCESS, which
> > may be used to assert properties of synchronization logic, where
> > violation cannot be detected as a normal data race.
> >
> > Examples of the reports that may be generated:
> >
> >     ==================================================================
> >     BUG: KCSAN: data-race in test_thread / test_thread
> >
> >     write to 0xffffffffab3d1540 of 8 bytes by task 466 on cpu 2:
> >      test_thread+0x8d/0x111
> >      debugfs_write.cold+0x32/0x44
> >      ...
> >
> >     assert no writes to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
> >      test_thread+0xa3/0x111
> >      debugfs_write.cold+0x32/0x44
> >      ...
> >     ==================================================================
> >
> >     ==================================================================
> >     BUG: KCSAN: data-race in test_thread / test_thread
> >
> >     assert no accesses to 0xffffffffab3d1540 of 8 bytes by task 465 on cpu 1:
> >      test_thread+0xb9/0x111
> >      debugfs_write.cold+0x32/0x44
> >      ...
> >
> >     read to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
> >      test_thread+0x77/0x111
> >      debugfs_write.cold+0x32/0x44
> >      ...
> >     ==================================================================
> >
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> >
> > Please let me know if the names make sense, given they do not include a
> > KCSAN_ prefix.
>
> I am OK with this, but there might well be some bikeshedding later on.
> Which should not be a real problem, irritating though it might be.
>
> > The names are unique across the kernel. I wouldn't expect another macro
> > with the same name but different semantics to pop up any time soon. If
> > there is a dual use to these macros (e.g. another tool that could hook
> > into it), we could also move it elsewhere (include/linux/compiler.h?).
> >
> > We can also revisit the original suggestion of WRITE_ONCE_EXCLUSIVE(),
> > if it is something that'd be used very widely. It'd be straightforward
> > to add with the help of these macros, but would need to be added to
> > include/linux/compiler.h.
>
> A more definite use case for ASSERT_EXCLUSIVE_ACCESS() is a
> reference-counting algorithm where exclusive access is expected after
> a successful atomic_dec_and_test().  Any objection to making the
> docbook header use that example?  I believe that a more familiar
> example would help people see the point of all this.  ;-)

Happy to update the example -- I'll send it tomorrow.

> I am queueing these as-is for review and testing, but please feel free
> to send updated versions.  Easy to do the replacement!

Thank you!

> And you knew that this was coming...  It looks to me that I can
> do something like this:
>
>         struct foo {
>                 int a;
>                 char b;
>                 long c;
>                 atomic_t refctr;
>         };
>
>         void do_a_foo(struct foo *fp)
>         {
>                 if (atomic_dec_and_test(&fp->refctr)) {
>                         ASSERT_EXCLUSIVE_ACCESS(*fp);
>                         safely_dispose_of(fp);
>                 }
>         }
>
> Does that work, or is it necessary to assert for each field separately?

That works just fine, and will check for races on the whole struct.

Thanks,
-- Marco

>                                                         Thanx, Paul
>
> > ---
> >  include/linux/kcsan-checks.h | 34 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 34 insertions(+)
> >
> > diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> > index 21b1d1f214ad5..1a7b51e516335 100644
> > --- a/include/linux/kcsan-checks.h
> > +++ b/include/linux/kcsan-checks.h
> > @@ -96,4 +96,38 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> >       kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
> >  #endif
> >
> > +/**
> > + * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
> > + *
> > + * Assert that there are no other threads writing @var; other readers are
> > + * allowed. This assertion can be used to specify properties of synchronization
> > + * logic, where violation cannot be detected as a normal data race.
> > + *
> > + * For example, if a per-CPU variable is only meant to be written by a single
> > + * CPU, but may be read from other CPUs; in this case, reads and writes must be
> > + * marked properly, however, if an off-CPU WRITE_ONCE() races with the owning
> > + * CPU's WRITE_ONCE(), would not constitute a data race but could be a harmful
> > + * race condition. Using this macro allows specifying this property in the code
> > + * and catch such bugs.
> > + *
> > + * @var variable to assert on
> > + */
> > +#define ASSERT_EXCLUSIVE_WRITER(var)                                           \
> > +     __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
> > +
> > +/**
> > + * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
> > + *
> > + * Assert that no other thread is accessing @var (no readers nor writers). This
> > + * assertion can be used to specify properties of synchronization logic, where
> > + * violation cannot be detected as a normal data race.
> > + *
> > + * For example, if a variable is not read nor written by the current thread, nor
> > + * should it be touched by any other threads during the current execution phase.
> > + *
> > + * @var variable to assert on
> > + */
> > +#define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
> > +     __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
> > +
> >  #endif /* _LINUX_KCSAN_CHECKS_H */
> > --
> > 2.25.0.341.g760bfbb309-goog
> >
>
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