> On Jul 17, 2019, at 4:59 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Jul 17, 2019 at 10:39:44AM +0200, Peter Zijlstra wrote:
>> On Tue, Jul 16, 2019 at 08:47:24PM +0200, Peter Zijlstra wrote:
>
>>> My primary concern was readability; I find the above suggestion much
>>> more readable. Maybe it can be written differently; you'll have to play
>>> around a bit.
>>
>> static void cna_splice_tail(struct cna_node *cn, struct cna_node *head, struct cna_node *tail)
>> {
>> struct cna_node *list;
>>
>> /* remove [head,tail] */
>> WRITE_ONCE(cn->mcs.next, tail->mcs.next);
>> tail->mcs.next = NULL;
>>
>> /* stick [head,tail] on the secondary list tail */
>> if (cn->mcs.locked <= 1) {
>> /* create secondary list */
>> head->tail = tail;
>> cn->mcs.locked = head->encoded_tail;
>> } else {
>> /* add to tail */
>> list = (struct cna_node *)decode_tail(cn->mcs.locked);
>> list->tail->next = head;
>> list->tail = tail;
>> }
>> }
>>
>> static struct cna_node *cna_find_next(struct mcs_spinlock *node)
>> {
>> struct cna_node *cni, *cn = (struct cna_node *)node;
>> struct cna_node *head, *tail = NULL;
>>
>> /* find any next lock from 'our' node */
>> for (head = cni = (struct cna_node *)READ_ONCE(cn->mcs.next);
>> cni && cni->node != cn->node;
>> tail = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next))
>> ;
>
> I think we can do away with those READ_ONCE()s, at this point those
> pointers should be stable. But please double check.
I think we can get rid of WRITE_ONCE above and the first READ_ONCE, as the
“first” next pointer (in the node of the current lock holder) is stable at this
point, and is not read / written concurrently. We do need the second READ_ONCE
as we traverse the list and can come across a next pointer being changed.
— Alex
>
>> /* when found, splice any skipped locks onto the secondary list */
>> if (cni && tail)
>> cna_splice_tail(cn, head, tail);
>>
>> return cni;
>> }
>>
>> How's that?
