linux-kernel - Re: [PATCH v3 1/9] kcsan: Add Kernel Concurrency Sanitizer infrastructure

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Date:   Fri, 8 Nov 2019 15:23:31 +0100
From:   Marco Elver <elver@...gle.com>
To:     Bhupesh Sharma <bhsharma@...hat.com>
Cc:     akiyks@...il.com, stern@...land.harvard.edu,
        Alexander Potapenko <glider@...gle.com>,
        parri.andrea@...il.com, andreyknvl@...gle.com,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, boqun.feng@...il.com,
        Borislav Petkov <bp@...en8.de>, dja@...ens.net,
        dlustig@...dia.com, Dave Hansen <dave.hansen@...ux.intel.com>,
        David Howells <dhowells@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        j.alglave@....ac.uk, joel@...lfernandes.org,
        Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>, luc.maranget@...ia.fr,
        Mark Rutland <mark.rutland@....com>, npiggin@...il.com,
        paulmck@...nel.org, Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, kasan-dev@...glegroups.com,
        linux-arch@...r.kernel.org,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-efi@...r.kernel.org, linux-kbuild@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH v3 1/9] kcsan: Add Kernel Concurrency Sanitizer
 infrastructure

Hi Bhupesh,

Thanks for your comments, see answers below.

On Fri, 08 Nov 2019, Bhupesh Sharma wrote:

> Sorry for the late comments, but I am just trying to understand the
> new KCSAN feature (which IMO seems very useful for debugging issues).
> 
> Some comments inline:
> 
> On Mon, Nov 4, 2019 at 7:59 PM Marco Elver <elver@...gle.com> wrote:
> >
...
> > diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
> > new file mode 100644
> > index 000000000000..bd8122acae01
> > --- /dev/null
> > +++ b/include/linux/kcsan.h
> > @@ -0,0 +1,115 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _LINUX_KCSAN_H
> > +#define _LINUX_KCSAN_H
> > +
> > +#include <linux/types.h>
> > +#include <linux/kcsan-checks.h>
> 
> For the new changes introduced (especially the new header files), can
> we please try to keep the alphabetical order
> for the include'd files.
> 
> The same comment applies for changes below ...

Done for v4.

...
> > +void kcsan_disable_current(void)
> > +{
> > +       ++get_ctx()->disable_count;
> > +}
> > +EXPORT_SYMBOL(kcsan_disable_current);
> > +
> > +void kcsan_enable_current(void)
> > +{
> > +       if (get_ctx()->disable_count-- == 0) {
> > +               kcsan_disable_current(); /* restore to 0 */
> > +               kcsan_disable_current();
> > +               WARN(1, "mismatching %s", __func__);
> 
> I am not sure I understand, why we need to call
> 'kcsan_disable_current()' twice and what the WARN message conveys.
> May-be you can add a comment here, or a more descriptive WARN meesage.

This branch is entered when there is an imbalance between
kcsan_disable_current and kcsan_enable_current calls. When entering the
branch, the decrement transitioned disable_count to -1, which should not
happen. The call to kcsan_disable_current restores it to 0, and the
following kcsan_disable_current actually disables KCSAN for generating
the warning.

> > +               kcsan_enable_current();
> > +       }
> > +}
> > +EXPORT_SYMBOL(kcsan_enable_current);
> > +
> > +void kcsan_nestable_atomic_begin(void)
> > +{
> > +       /*
> > +        * Do *not* check and warn if we are in a flat atomic region: nestable
> > +        * and flat atomic regions are independent from each other.
> > +        * See include/linux/kcsan.h: struct kcsan_ctx comments for more
> > +        * comments.
> > +        */
> > +
> > +       ++get_ctx()->atomic_nest_count;
> > +}
> > +EXPORT_SYMBOL(kcsan_nestable_atomic_begin);
> > +
> > +void kcsan_nestable_atomic_end(void)
> > +{
> > +       if (get_ctx()->atomic_nest_count-- == 0) {
> > +               kcsan_nestable_atomic_begin(); /* restore to 0 */
> > +               kcsan_disable_current();
> > +               WARN(1, "mismatching %s", __func__);
> 
> .. Same as above.

Same situation, except for atomic_nest_count. Here also
atomic_nest_count is -1 which should not happen.

I've added some more comments.

> > +               kcsan_enable_current();
> > +       }
> > +}
> > +EXPORT_SYMBOL(kcsan_nestable_atomic_end);

Best Wishes,
-- Marco