Re: [PATCH for 2.5] preemptible kernel

Rusty Russell
2001-03-30T20:26:27Z

In message <Pine.LNX.4.05.10103291555390.8122-100000@cosmic.nrg.org> you write:
> Here is an attempt at a possible version of synchronize_kernel() that
> should work on a preemptible kernel. I haven't tested it yet.

It's close, but...

Those who suggest that we don't do preemtion on SMP make this much
easier (synchronize_kernel() is a NOP on UP), and I'm starting to
agree with them. Anyway:

> if (p->state == TASK_RUNNING ||
> (p->state == (TASK_RUNNING|TASK_PREEMPTED))) {
> p->flags |= PF_SYNCING;

Setting a running task's flags brings races, AFAICT, and checking
p->state is NOT sufficient, consider wait_event(): you need p->has_cpu
here I think. You could do it for TASK_PREEMPTED only, but you'd have
to do the "unreal priority" part of synchronize_kernel() with some
method to say "don't preempt anyone", but it will hurt latency.
Hmmm...

The only way I can see is to have a new element in "struct
task_struct" saying "syncing now", which is protected by the runqueue
lock. This looks like (and I prefer wait queues, they have such nice
helpers):

static DECLARE_WAIT_QUEUE_HEAD(syncing_task);
static DECLARE_MUTEX(synchronize_kernel_mtx);
static int sync_count = 0;

schedule():
if (!(prev->state & TASK_PREEMPTED) && prev->syncing)
if (--sync_count == 0) wake_up(&syncing_task);

synchronize_kernel():
{
struct list_head *tmp;
struct task_struct *p;

/* Guard against multiple calls to this function */
down(&synchronize_kernel_mtx);

/* Everyone running now or currently preempted must
voluntarily schedule before we know we are safe. */
spin_lock_irq(&runqueue_lock);
list_for_each(tmp, &runqueue_head) {
p = list_entry(tmp, struct task_struct, run_list);
if (p->has_cpu || p->state == (TASK_RUNNING|TASK_PREEMPTED)) {
p->syncing = 1;
sync_count++;
}
}
spin_unlock_irq(&runqueue_lock);

/* Wait for them all */
wait_event(syncing_task, sync_count == 0);
up(&synchronize_kernel_mtx);
}

Also untested 8),
Rusty.
--
Premature optmztion is rt of all evl. --DK