Re: [PATCH v6 2/4] rcu/segcblist: Add counters to segcblist datastructure

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Sep 23, 2020 at 11:22:09AM -0400, Joel Fernandes (Google) wrote:
> +/* Return number of callbacks in a segment of the segmented callback list. */
> +static void rcu_segcblist_add_seglen(struct rcu_segcblist *rsclp, int seg, long v)
> +{
> +#ifdef CONFIG_RCU_NOCB_CPU
> +	smp_mb__before_atomic(); /* Up to the caller! */
> +	atomic_long_add(v, &rsclp->seglen[seg]);
> +	smp_mb__after_atomic(); /* Up to the caller! */
> +#else
> +	smp_mb(); /* Up to the caller! */
> +	WRITE_ONCE(rsclp->seglen[seg], rsclp->seglen[seg] + v);
> +	smp_mb(); /* Up to the caller! */
> +#endif
> +}

I know that these "Up to the caller" comments come from the existing len
functions but perhaps we should explain a bit more against what it is ordering
and what it pairs to.

Also why do we need one before _and_ after?

And finally do we have the same ordering requirements than the unsegmented len
field?

> +
> +/* Move from's segment length to to's segment. */
> +static void rcu_segcblist_move_seglen(struct rcu_segcblist *rsclp, int from, int to)
> +{
> +	long len;
> +
> +	if (from == to)
> +		return;
> +
> +	len = rcu_segcblist_get_seglen(rsclp, from);
> +	if (!len)
> +		return;
> +
> +	rcu_segcblist_add_seglen(rsclp, to, len);
> +	rcu_segcblist_set_seglen(rsclp, from, 0);
> +}
> +
[...]
> @@ -245,6 +283,7 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
>  			   struct rcu_head *rhp)
>  {
>  	rcu_segcblist_inc_len(rsclp);
> +	rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
>  	smp_mb(); /* Ensure counts are updated before callback is enqueued. */

Since inc_len and even now inc_seglen have two full barriers embracing the add up,
we can probably spare the above smp_mb()?

>  	rhp->next = NULL;
>  	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
> @@ -274,27 +313,13 @@ bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
>  	for (i = RCU_NEXT_TAIL; i > RCU_DONE_TAIL; i--)
>  		if (rsclp->tails[i] != rsclp->tails[i - 1])
>  			break;
> +	rcu_segcblist_inc_seglen(rsclp, i);
>  	WRITE_ONCE(*rsclp->tails[i], rhp);
>  	for (; i <= RCU_NEXT_TAIL; i++)
>  		WRITE_ONCE(rsclp->tails[i], &rhp->next);
>  	return true;
>  }
>  
> @@ -403,6 +437,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
>  		if (ULONG_CMP_LT(seq, rsclp->gp_seq[i]))
>  			break;
>  		WRITE_ONCE(rsclp->tails[RCU_DONE_TAIL], rsclp->tails[i]);
> +		rcu_segcblist_move_seglen(rsclp, i, RCU_DONE_TAIL);

Do we still need the same amount of full barriers contained in add() called by move() here?
It's called in the reverse order (write queue then len) than usual. If I trust the comment
in rcu_segcblist_enqueue(), the point of the barrier is to make the length visible before
the new callback for rcu_barrier() (although that concerns len and not seglen). But here
above, the unsegmented length doesn't change. I could understand a write barrier between
add_seglen(x, i) and set_seglen(0, RCU_DONE_TAIL) but I couldn't find a paired couple either.

>  	}
>  
>  	/* If no callbacks moved, nothing more need be done. */
> @@ -423,6 +458,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
>  		if (rsclp->tails[j] == rsclp->tails[RCU_NEXT_TAIL])
>  			break;  /* No more callbacks. */
>  		WRITE_ONCE(rsclp->tails[j], rsclp->tails[i]);
> +		rcu_segcblist_move_seglen(rsclp, i, j);

Same question here (feel free to reply "same answer" :o)

Thanks!