Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

Frederic Weisbecker
2024-01-27T22:54:49Z

Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit :
> +static bool tmigr_inactive_up(struct tmigr_group *group,
> + struct tmigr_group *child,
> + void *ptr)
> +{
> + union tmigr_state curstate, newstate, childstate;
> + struct tmigr_walk *data = ptr;
> + bool walk_done;
> + u8 childmask;
> +
> + childmask = data->childmask;
> + curstate.state = atomic_read(&group->migr_state);
> + childstate.state = 0;
> +
> + do {

So I got the confirmation from Boqun (+Cc) and Paul that a failing cmpxchg
may not order the load of the old value against subsequent loads. And
that may apply to atomic_try_cmpxchg() as well.

Therefore you not only need to turn group->migr_state read into
an atomic_read_acquire() but you also need to do this on each iteration
of this loop. For example you can move the read_acquire right here.

Thanks.

> + if (child)
> + childstate.state = atomic_read(&child->migr_state);
> +
> + newstate = curstate;
> + walk_done = true;
> +
> + /* Reset active bit when the child is no longer active */
> + if (!childstate.active)
> + newstate.active &= ~childmask;
> +
> + if (newstate.migrator == childmask) {
> + /*
> + * Find a new migrator for the group, because the child
> + * group is idle!
> + */
> + if (!childstate.active) {
> + unsigned long new_migr_bit, active = newstate.active;
> +
> + new_migr_bit = find_first_bit(&active, BIT_CNT);
> +
> + if (new_migr_bit != BIT_CNT) {
> + newstate.migrator = BIT(new_migr_bit);
> + } else {
> + newstate.migrator = TMIGR_NONE;
> +
> + /* Changes need to be propagated */
> + walk_done = false;
> + }
> + }
> + }
> +
> + newstate.seq++;
> +
> + WARN_ON_ONCE((newstate.migrator != TMIGR_NONE) && !(newstate.active));
> +
> + } while (!atomic_try_cmpxchg(&group->migr_state, &curstate.state, newstate.state));