Re: [PATCH kcsan 27/32] kcsan: Add option to allow watcher interruptions

[Marco Elver]

On Thu, 12 Mar 2020 at 19:04, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Thu, Mar 12, 2020 at 11:03:28AM -0700, Paul E. McKenney wrote:
> > On Mon, Mar 09, 2020 at 12:04:15PM -0700, paulmck@xxxxxxxxxx wrote:
> > > From: Marco Elver <elver@xxxxxxxxxx>
> > >
> > > Add option to allow interrupts while a watchpoint is set up. This can be
> > > enabled either via CONFIG_KCSAN_INTERRUPT_WATCHER or via the boot
> > > parameter 'kcsan.interrupt_watcher=1'.
> > >
> > > Note that, currently not all safe per-CPU access primitives and patterns
> > > are accounted for, which could result in false positives. For example,
> > > asm-generic/percpu.h uses plain operations, which by default are
> > > instrumented. On interrupts and subsequent accesses to the same
> > > variable, KCSAN would currently report a data race with this option.
> > >
> > > Therefore, this option should currently remain disabled by default, but
> > > may be enabled for specific test scenarios.
> > >
> > > To avoid new warnings, changes all uses of smp_processor_id() to use the
> > > raw version (as already done in kcsan_found_watchpoint()). The exact SMP
> > > processor id is for informational purposes in the report, and
> > > correctness is not affected.
> > >
> > > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> >
> > And I get silent hangs that bisect to this patch when running the
> > following rcutorture command, run in the kernel source tree on a
> > 12-hardware-thread laptop:
> >
> > bash tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 12 --duration 10 --kconfig "CONFIG_DEBUG_INFO=y CONFIG_KCSAN=y CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=n CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n CONFIG_KCSAN_REPORT_ONCE_IN_MS=100000 CONFIG_KCSAN_VERBOSE=y CONFIG_KCSAN_INTERRUPT_WATCHER=y" --configs TREE03
> >
> > It works fine on some (but not all) of the other rcutorture test
> > scenarios.  It fails on TREE01, TREE02, TREE03, TREE09.  The common thread
> > is that these are the TREE scenarios are all PREEMPT=y.  So are RUDE01,
> > SRCU-P, TASKS01, and TASKS03, but these scenarios are not hammering
> > on Tree RCU, and thus have far less interrupt activity and the like.
> > Given that it is an interrupt-related feature being added by this commit,
> > this seems like expected (mis)behavior.
> >
> > Can you reproduce this?  If not, are there any diagnostics I can add to
> > my testing?  Or a diagnostic patch I could apply?

I think I can reproduce it.  Let me debug some more, so far I haven't
found anything yet.

What I do know is that it's related to reporting. Turning kcsan_report
into a noop makes the test run to completion.

> I should hasten to add that this feature was quite helpful in recent work!

Good to know. :-)  We can probably keep this patch, since the default
config doesn't turn this on. But I will try to see what's up with the
hangs, and hopefully find a fix.

Thanks,
-- Marco