Re: [PATCH v5 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

Waiman Long
2019-10-18T16:03:13Z

On 10/16/19 12:29 AM, Alex Kogan wrote:
> +static inline void cna_pass_lock(struct mcs_spinlock *node,
> + struct mcs_spinlock *next)
> +{
> + struct cna_node *cn = (struct cna_node *)node;
> + struct mcs_spinlock *next_holder = next, *tail_2nd;
> + u32 val = 1;
> +
> + u32 scan = cn->pre_scan_result;
> +
> + /*
> + * check if a successor from the same numa node has not been found in
> + * pre-scan, and if so, try to find it in post-scan starting from the
> + * node where pre-scan stopped (stored in @pre_scan_result)
> + */
> + if (scan > 0)
> + scan = cna_scan_main_queue(node, decode_tail(scan));
> +
> + if (!scan) { /* if found a successor from the same numa node */
> + next_holder = node->next;
> + /*
> + * make sure @val gets 1 if current holder's @locked is 0 as
> + * we have to store a non-zero value in successor's @locked
> + * to pass the lock
> + */
> + val = node->locked + (node->locked == 0);

node->locked can be 0 when the cpu enters into an empty MCS queue. We
could unconditionally set node->locked to 1 for this case in qspinlock.c
or with your above code. Perhaps, a comment about when node->locked will
be 0.

It may be easier to understand if you just do

val = node->locked ? node->locked : 1;

Cheers,
Longman