Re: [PATCH v7 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

Peter Zijlstra
2020-01-23T09:01:13Z

On Wed, Jan 22, 2020 at 06:04:56PM +0100, Peter Zijlstra wrote:
> +/*
> + * cna_splice_head -- splice the entire secondary queue onto the head of the
> + * primary queue.
> + */
> +static cna_splice_head(struct qspinlock *lock, u32 val,
> + struct mcs_spinlock *node, struct mcs_spinlock *next)
> +{
> + struct mcs_spinlock *head_2nd, *tail_2nd;
> +
> + tail_2nd = decode_tail(node->locked);
> + head_2nd = tail_2nd->next;
> +
> + if (lock) {

That should be: if (!next) {

> + u32 new = ((struct cna_node *)tail_2nd)->encoded_tail | _Q_LOCKED_VAL;
> + if (!atomic_try_cmpxchg_relaxed(&lock->val, &val, new))
> + return NULL;
> +
> + /*
> + * The moment we've linked the primary tail we can race with
> + * the WRITE_ONCE(prev->next, node) store from new waiters.
> + * That store must win.
> + */

And that still is a shit comment; I'll go try again.

> + cmpxchg_relaxed(&tail_2nd->next, head_2nd, next);
> + } else {
> + tail_2nd->next = next;
> + }
> +
> + return head_2nd;
> +}