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!