Re: [PATCH RFC] sched: introduce add_wait_queue_exclusive_head

[Oleg Nesterov <oleg@redhat.com>]

On 03/19, Peter Zijlstra wrote:
>
> On Wed, Mar 19, 2014 at 05:49:07PM +0100, Oleg Nesterov wrote:
> > +static void add_wait_queue_flag(wait_queue_head_t *q, wait_queue_t *wait)
> > +{
> > +	struct list_head *head = &q->task_list;
> > +	wait_queue_t *excl;
> > +
> > +	if (wait->flags & WQ_FLAG_EXCLUSIVE) {
> > +		if (wait->flags & WQ_FLAG_EXCLUSIVE_HEAD) {
> > +			list_for_each_entry(excl, head, task_list)
> > +				if (excl->flags & WQ_FLAG_EXCLUSIVE) {
> > +					head = &excl->task_list;
> > +					break;
> > +				}
>
> I prefer an extra pair of { } here,

OK,

> but the main concern would be the
> cost of that iteration.

Yes.

This change assumes that we do not mix exclusive and !exclusive, in
this case list_for_each_entry() is cheap, the list is either empty
or the first entry is WQ_FLAG_EXCLUSIVE.

Otherwise the user should blame itself, but the code still will work
correctly.

Or we can do

	if (WQ_FLAG_EXCLUSIVE_HEAD) {
		WARN_ON(!list_empty(head) &&
			(list_first_entry(...)-flags & WQ_FLAG_EXCLUSIVE));
		...
	}

> Other than that, yes something like that would do I suppose.

OK, I'll try to test/cleanup/resend tomorrow.

Oleg.