Re: [PATCH] bitops, kcsan: Partially revert instrumentation for non-atomic bitops

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Aug 18, 2020 at 10:34:28AM +0200, Marco Elver wrote:
> On Thu, 13 Aug 2020 at 18:39, Marco Elver <elver@google.com> wrote:
> > Previous to the change to distinguish read-write accesses, when
> > CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y is set, KCSAN would consider
> > the non-atomic bitops as atomic. We want to partially revert to this
> > behaviour, but with one important distinction: report racing
> > modifications, since lost bits due to non-atomicity are certainly
> > possible.
> >
> > Given the operations here only modify a single bit, assuming
> > non-atomicity of the writer is sufficient may be reasonable for certain
> > usage (and follows the permissible nature of the "assume plain writes
> > atomic" rule). In other words:
> >
> >         1. We want non-atomic read-modify-write races to be reported;
> >            this is accomplished by kcsan_check_read(), where any
> >            concurrent write (atomic or not) will generate a report.
> >
> >         2. We do not want to report races with marked readers, but -do-
> >            want to report races with unmarked readers; this is
> >            accomplished by the instrument_write() ("assume atomic
> >            write" with Kconfig option set).
> >
> > With the above rules, when KCSAN_ASSUME_PLAIN_WRITES_ATOMIC is selected,
> > it is hoped that KCSAN's reporting behaviour is better aligned with
> > current expected permissible usage for non-atomic bitops.
> >
> > Note that, a side-effect of not telling KCSAN that the accesses are
> > read-writes, is that this information is not displayed in the access
> > summary in the report. It is, however, visible in inline-expanded stack
> > traces. For now, it does not make sense to introduce yet another special
> > case to KCSAN's runtime, only to cater to the case here.
> >
> > Signed-off-by: Marco Elver <elver@google.com>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Will Deacon <will@kernel.org>
> > ---
> > As discussed, partially reverting behaviour for non-atomic bitops when
> > KCSAN_ASSUME_PLAIN_WRITES_ATOMIC is selected.
> >
> > I'd like to avoid more special cases in KCSAN's runtime to cater to
> > cases like this, not only because it adds more complexity, but it
> > invites more special cases to be added. If there are other such
> > primitives, we likely have to do it on a case-by-case basis as well, and
> > justify carefully for each such case. But currently, as far as I can
> > tell, the bitops are truly special, simply because we do know each op
> > just touches a single bit.
> > ---
> >  .../bitops/instrumented-non-atomic.h          | 30 +++++++++++++++++--
> >  1 file changed, 27 insertions(+), 3 deletions(-)
> 
> Paul, if it looks good to you, feel free to pick it up.

Queued, thank you!

							Thanx, Paul