Re: [PATCH] kcsan, seqlock: Support seqcount_latch_t

[Peter Zijlstra <peterz@infradead.org>]

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?

Also, I note that update_clock_read_data() seems to do things
'backwards' and will completely elide your proposed annotation.

> 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.