question about blk_schedule_flush_plug

I was looking at the scheduler changes going into 2.6.39 again and wondered about the use of blk_schedule_flush_plug smack in the middle of schedule()

It looks like this:

			if (blk_needs_flush_plug(prev)) {
				raw_spin_unlock(&rq->lock);
				blk_schedule_flush_plug(prev);
				raw_spin_lock(&rq->lock);
			}

Now call me suspicious but to my eyes it looks really dubious unlocking the runqueue like that right in the heart of schedule().

Comments?

Thanks,
Con

-ck

Yeah, that's quite all-right, both pre_schedule() and idle_balance() already did that too.

We can safely release the runqueue lock after deactivate_task() and before put_prev_task().

Releasing/retaking rq->lock is nothing new:

static void idle_balance(int this_cpu, struct rq *this_rq) {
...

        /*
         * Drop the rq->lock, but keep IRQ/preempt disabled.
         */
        raw_spin_unlock(&this_rq->lock);

See also need_resched, and double_lock_balance() instances.

-Mike