Re: [PATCH] kcsan, seqlock: Support seqcount_latch_t

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 29, 2024 at 02:05:38PM +0100, Marco Elver wrote:
> On Tue, 29 Oct 2024 at 12:49, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Oct 29, 2024 at 09:36:29AM +0100, Marco Elver wrote:
> > > Reviewing current raw_write_seqcount_latch() callers, the most common
> > > patterns involve only few memory accesses, either a single plain C
> > > assignment, or memcpy;
> >
> > Then I assume you've encountered latch_tree_{insert,erase}() in your
> > travels, right?
> 
> Oops. That once certainly exceeds the "8 memory accesses".
> 
> > Also, I note that update_clock_read_data() seems to do things
> > 'backwards' and will completely elide your proposed annotation.
> 
> Hmm, for the first access, yes. This particular oddity could be
> "fixed" by surrounding the accesses by
> kcsan_nestable_atomic_begin/end(). I don't know if it warrants adding
> a raw_write_seqcount_latch_begin().
> 
> Preferences?

I *think* it is doable to flip it around to the 'normal' order, but
given I've been near cross-eyed with a head-ache these past two days,
I'm not going to attempt a patch for you, since I'm bound to get it
wrong :/

> > > therefore, the value of 8 memory accesses after
> > > raw_write_seqcount_latch() is chosen to (a) avoid most false positives,
> > > and (b) avoid excessive number of false negatives (due to inadvertently
> > > declaring most accesses in the proximity of update_fast_timekeeper() as
> > > "atomic").
> >
> > The above latch'ed RB-trees can certainly exceed this magical number 8.
> >
> > > Reported-by: Alexander Potapenko <glider@google.com>
> > > Tested-by: Alexander Potapenko <glider@google.com>
> > > Fixes: 88ecd153be95 ("seqlock, kcsan: Add annotations for KCSAN")
> > > Signed-off-by: Marco Elver <elver@google.com>
> > > ---
> > >  include/linux/seqlock.h | 9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > >
> > > diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> > > index fffeb754880f..e24cf144276e 100644
> > > --- a/include/linux/seqlock.h
> > > +++ b/include/linux/seqlock.h
> > > @@ -614,6 +614,7 @@ typedef struct {
> > >   */
> > >  static __always_inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
> > >  {
> > > +     kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
> > >       /*
> > >        * Pairs with the first smp_wmb() in raw_write_seqcount_latch().
> > >        * Due to the dependent load, a full smp_rmb() is not needed.
> > > @@ -631,6 +632,7 @@ static __always_inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *
> > >  static __always_inline int
> > >  raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
> > >  {
> > > +     kcsan_atomic_next(0);
> > >       smp_rmb();
> > >       return unlikely(READ_ONCE(s->seqcount.sequence) != start);
> > >  }
> > > @@ -721,6 +723,13 @@ static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
> > >       smp_wmb();      /* prior stores before incrementing "sequence" */
> > >       s->seqcount.sequence++;
> > >       smp_wmb();      /* increment "sequence" before following stores */
> > > +
> > > +     /*
> > > +      * Latch writers do not have a well-defined critical section, but to
> > > +      * avoid most false positives, at the cost of false negatives, assume
> > > +      * the next few memory accesses belong to the latch writer.
> > > +      */
> > > +     kcsan_atomic_next(8);
> > >  }
> >
> > Given there are so very few latch users, would it make sense to
> > introduce a raw_write_seqcount_latch_end() callback that does
> > kcsan_atomic_next(0) ? -- or something along those lines? Then you won't
> > have to assume such a small number.
> 
> That's something I considered, but thought I'd try the unintrusive
> version first. But since you proposed it here, I'd much prefer that,
> too. ;-)
> Let me try that.
> 
> Thanks,
> -- Marco