From: Mark Rutland <mark.rutland@arm.com>
To: Marco Elver <elver@google.com>
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>,
Andrey Konovalov <andreyknvl@google.com>,
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>,
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Dmitry Vyukov <dvyukov@google.com>,
"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
Jade Alglave <j.alglave@ucl.ac.uk>,
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Nicholas Piggin <npiggin@gmail.com>,
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Subject: Re: [PATCH v2 4/8] seqlock, kcsan: Add annotations for KCSAN
Date: Thu, 24 Oct 2019 17:35:46 +0100 [thread overview]
Message-ID: <20191024163545.GI4300@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <CANpmjNPFkqOSEcEP475-NeeJnY5pZ44m+bEhtOs8E_xkRKr-TQ@mail.gmail.com>
On Thu, Oct 24, 2019 at 04:17:11PM +0200, Marco Elver wrote:
> On Thu, 24 Oct 2019 at 14:28, Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > On Thu, Oct 17, 2019 at 04:13:01PM +0200, Marco Elver wrote:
> > > Since seqlocks in the Linux kernel do not require the use of marked
> > > atomic accesses in critical sections, we teach KCSAN to assume such
> > > accesses are atomic. KCSAN currently also pretends that writes to
> > > `sequence` are atomic, although currently plain writes are used (their
> > > corresponding reads are READ_ONCE).
> > >
> > > Further, to avoid false positives in the absence of clear ending of a
> > > seqlock reader critical section (only when using the raw interface),
> > > KCSAN assumes a fixed number of accesses after start of a seqlock
> > > critical section are atomic.
> >
> > Do we have many examples where there's not a clear end to a seqlock
> > sequence? Or are there just a handful?
> >
> > If there aren't that many, I wonder if we can make it mandatory to have
> > an explicit end, or to add some helper for those patterns so that we can
> > reliably hook them.
>
> In an ideal world, all usage of seqlocks would be via seqlock_t, which
> follows a somewhat saner usage, where we already do normal begin/end
> markings -- with subtle exception to readers needing to be flat atomic
> regions, e.g. because usage like this:
> - fs/namespace.c:__legitimize_mnt - unbalanced read_seqretry
> - fs/dcache.c:d_walk - unbalanced need_seqretry
>
> But anything directly accessing seqcount_t seems to be unpredictable.
> Filtering for usage of read_seqcount_retry not following 'do { .. }
> while (read_seqcount_retry(..));' (although even the ones in while
> loops aren't necessarily predictable):
>
> $ git grep 'read_seqcount_retry' | grep -Ev 'seqlock.h|Doc|\* ' | grep
> -v 'while ('
> => about 1/3 of the total read_seqcount_retry usage.
>
> Just looking at fs/namei.c, I would conclude that it'd be a pretty
> daunting task to prescribe and migrate to an interface that forces
> clear begin/end.
>
> Which is why I concluded that for now, it is probably better to make
> KCSAN play well with the existing code.
Thanks for the detailed explanation, it's very helpful.
That all sounds reasonable to me -- could you fold some of that into the
commit message?
Thanks,
Mark.
next prev parent reply other threads:[~2019-10-24 16:36 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-17 14:12 [PATCH v2 0/8] Add Kernel Concurrency Sanitizer (KCSAN) Marco Elver
2019-10-17 14:12 ` [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure Marco Elver
2019-10-21 13:37 ` Alexander Potapenko
2019-10-21 15:54 ` Marco Elver
2019-10-22 14:11 ` Mark Rutland
2019-10-22 16:52 ` Marco Elver
2019-10-22 15:48 ` Oleg Nesterov
2019-10-22 17:42 ` Marco Elver
2019-10-23 16:24 ` Oleg Nesterov
2019-10-24 11:02 ` Marco Elver
2019-10-23 9:41 ` Dmitry Vyukov
2019-10-23 9:56 ` Dmitry Vyukov
2019-10-23 10:03 ` Dmitry Vyukov
2019-10-23 10:09 ` Dmitry Vyukov
2019-10-23 10:28 ` Dmitry Vyukov
2019-10-23 11:08 ` Dmitry Vyukov
2019-10-23 11:20 ` Dmitry Vyukov
2019-10-23 12:05 ` Dmitry Vyukov
2019-10-23 12:32 ` Dmitry Vyukov
2019-10-17 14:12 ` [PATCH v2 2/8] objtool, kcsan: Add KCSAN runtime functions to whitelist Marco Elver
2019-10-21 15:15 ` Dmitry Vyukov
2019-10-21 15:43 ` Marco Elver
2019-10-17 14:13 ` [PATCH v2 3/8] build, kcsan: Add KCSAN build exceptions Marco Elver
2019-10-17 14:13 ` [PATCH v2 4/8] seqlock, kcsan: Add annotations for KCSAN Marco Elver
2019-10-24 12:28 ` Mark Rutland
2019-10-24 14:17 ` Marco Elver
2019-10-24 16:35 ` Mark Rutland [this message]
2019-10-24 17:09 ` Marco Elver
2019-10-17 14:13 ` [PATCH v2 5/8] seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier Marco Elver
2019-10-17 14:13 ` [PATCH v2 6/8] asm-generic, kcsan: Add KCSAN instrumentation for bitops Marco Elver
2019-10-17 14:13 ` [PATCH v2 7/8] locking/atomics, kcsan: Add KCSAN instrumentation Marco Elver
2019-10-22 12:33 ` Mark Rutland
2019-10-22 18:17 ` Marco Elver
2019-10-17 14:13 ` [PATCH v2 8/8] x86, kcsan: Enable KCSAN for x86 Marco Elver
2019-10-22 12:59 ` Mark Rutland
2019-10-22 13:02 ` Marco Elver
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