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Date:   Thu, 12 Mar 2020 11:04:14 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
        kernel-team@fb.com, mingo@kernel.org
Cc:     elver@google.com, andreyknvl@google.com, glider@google.com,
        dvyukov@google.com, cai@lca.pw, boqun.feng@gmail.com
Subject: Re: [PATCH kcsan 27/32] kcsan: Add option to allow watcher
 interruptions
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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@kernel.org wrote:
> > From: Marco Elver <elver@google.com>
> > 
> > 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@google.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> 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 should hasten to add that this feature was quite helpful in recent work!

							Thanx, Paul