Re: [PATCH v4 00/10] Add Kernel Concurrency Sanitizer (KCSAN)

From: Marco Elver <elver@google.com>
To: Qian Cai <cai@lca.pw>
Cc: LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	Alexander Potapenko <glider@google.com>,
	Andrea Parri <parri.andrea@gmail.com>,
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	Andy Lutomirski <luto@kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>, Boqun Feng <boqun.feng@gmail.com>,
	Borislav Petkov <bp@alien8.de>, Daniel Axtens <dja@axtens.net>,
	Daniel Lustig <dlustig@nvidia.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Howells <dhowells@redhat.com>,
	Dmitry Vyukov <dvyukov@google.com>,
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Subject: Re: [PATCH v4 00/10] Add Kernel Concurrency Sanitizer (KCSAN)
Date: Tue, 19 Nov 2019 22:50:02 +0100	[thread overview]
Message-ID: <CANpmjNPynCwYc8-GKTreJ8HF81k14JAHZXLt0jQJr_d+ukL=6A@mail.gmail.com> (raw)
In-Reply-To: <1574194379.9585.10.camel@lca.pw>

On Tue, 19 Nov 2019 at 21:13, Qian Cai <cai@lca.pw> wrote:
>
> On Thu, 2019-11-14 at 19:02 +0100, 'Marco Elver' via kasan-dev wrote:
> > This is the patch-series for the Kernel Concurrency Sanitizer (KCSAN).
> > KCSAN is a sampling watchpoint-based *data race detector*. More details
> > are included in **Documentation/dev-tools/kcsan.rst**. This patch-series
> > only enables KCSAN for x86, but we expect adding support for other
> > architectures is relatively straightforward (we are aware of
> > experimental ARM64 and POWER support).
>
> This does not allow the system to boot. Just hang forever at the end.
>
> https://cailca.github.io/files/dmesg.txt
>
> the config (dselect KASAN and select KCSAN with default options):
>
> https://raw.githubusercontent.com/cailca/linux-mm/master/x86.config

Thanks! That config enables lots of other debug code. I could
reproduce the hang. It's related to CONFIG_PROVE_LOCKING etc.

The problem is definitely not the fact that kcsan_setup_watchpoint
disables interrupts (tested by removing that code). Although lockdep
still complains here, and looking at the code in kcsan/core.c, I just
can't see how local_irq_restore cannot be called before returning (in
the stacktrace you provided, there is no kcsan function), and
interrupts should always be re-enabled. (Interrupts are only disabled
during delay in kcsan_setup_watchpoint.)

What I also notice is that this happens when the console starts
getting spammed with data-race reports (presumably because some extra
debug code has lots of data races according to KCSAN).

My guess is that some of the extra debug logic enabled in that config
is incompatible with KCSAN. However, so far I cannot tell where
exactly the problem is. For now the work-around would be not using
KCSAN with these extra debug options.  I will investigate more, but
nothing obviously wrong stands out..

Many thanks,
-- Marco