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Date: Fri, 13 Mar 2020 16:28:23 +0100 From: Marco Elver <elver@...gle.com> To: "Paul E. McKenney" <paulmck@...nel.org> Cc: LKML <linux-kernel@...r.kernel.org>, kasan-dev <kasan-dev@...glegroups.com>, kernel-team@...com, Ingo Molnar <mingo@...nel.org>, Andrey Konovalov <andreyknvl@...gle.com>, Alexander Potapenko <glider@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Qian Cai <cai@....pw>, Boqun Feng <boqun.feng@...il.com> Subject: Re: [PATCH kcsan 27/32] kcsan: Add option to allow watcher interruptions On Thu, 12 Mar 2020 at 19:04, Paul E. McKenney <paulmck@...nel.org> 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@...nel.org wrote: > > > From: Marco Elver <elver@...gle.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@...gle.com> > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.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 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
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