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Date:   Thu, 24 Oct 2019 17:35:46 +0100
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>,
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Subject: Re: [PATCH v2 4/8] seqlock, kcsan: Add annotations for KCSAN
Message-ID: <20191024163545.GI4300@lakrids.cambridge.arm.com>
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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.