Re: [PATCH v7 6/6] rcu/segcblist: Add additional comments to explain smp_mb()

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Oct 14, 2020 at 08:23:01PM -0400, Joel Fernandes (Google) wrote:
> Memory barriers are needed when updating the full length of the
> segcblist, however it is not fully clearly why one is needed before and
> after. This patch therefore adds additional comments to the function
> header to explain it.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/rcu/rcu_segcblist.c | 38 ++++++++++++++++++++++++++++++++++----
>  1 file changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 271d5d9d7f60..25ffd07f9951 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -147,17 +147,47 @@ static void rcu_segcblist_inc_seglen(struct rcu_segcblist *rsclp, int seg)
>   * field to disagree with the actual number of callbacks on the structure.
>   * This increase is fully ordered with respect to the callers accesses
>   * both before and after.
> + *
> + * About memory barriers:
> + * There is a situation where rcu_barrier() locklessly samples the full
> + * length of the segmented cblist before deciding what to do. That can
> + * race with another path that calls this function. rcu_barrier() should
> + * not wrongly assume there are no callbacks, so any transitions from 1->0
> + * and 0->1 have to be carefully ordered with respect to list modifications.
> + *
> + * Memory barrier is needed before adding to length, for the case where
> + * v is negative which does not happen in current code, but used
> + * to happen. Keep the memory barrier for robustness reasons.

Heh, I seem to recongnize someone's decision's style ;-)

>     When/If the
> + * length transitions from 1 -> 0, the write to 0 has to be ordered *after*
> + * the memory accesses of the CBs that were dequeued and the segcblist
> + * modifications:
> + * P0 (what P1 sees)	P1
> + * set len = 0
> + *                      rcu_barrier sees len as 0
> + * dequeue from list
> + *                      rcu_barrier does nothing.

It's a bit difficult to read that way. So that would be:


      rcu_do_batch()                rcu_barrier()
      --                            --
      dequeue                       l = READ(len)
      smp_mb()                      if (!l)
      WRITE(len, 0)                     check next CPU...

But I'm a bit confused against what it pairs in rcu_barrier().

> + *
> + * Memory barrier is needed after adding to length for the case
> + * where length transitions from 0 -> 1. This is because rcu_barrier()
> + * should never miss an update to the length. So the update to length
> + * has to be seen *before* any modifications to the segmented list. Otherwise a
> + * race can happen.
> + * P0 (what P1 sees)	P1
> + * queue to list
> + *                      rcu_barrier sees len as 0
> + * set len = 1.
> + *                      rcu_barrier does nothing.

So that would be:

      call_rcu()                    rcu_barrier()
      --                            --
      WRITE(len, len + 1)           l = READ(len)
      smp_mb()                      if (!l)
      queue                            check next CPU...


But I still don't see against what it pairs in rcu_barrier.

Thanks.