Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [PATCH for 2.5] preemptible kernel | Date | Sat, 31 Mar 2001 06:11:39 +1000 |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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