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SubjectRe: [PATCH RFC] sched: introduce add_wait_queue_exclusive_head
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.



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