Messages in this thread | | | Date | Thu, 10 Jan 2002 00:10:02 -0500 | From | kevin@koconnor ... | Subject | lock order in O(1) scheduler |
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Hi Ingo,
I was looking through the new O(1) scheduler (found in linux-2.5.2-pre11), when I came upon the following code in try_to_wake_up():
lock_task_rq(rq, p, flags); p->state = TASK_RUNNING; if (!p->array) { if (!rt_task(p) && synchronous && (smp_processor_id() < p->cpu)) { spin_lock(&this_rq()->lock); p->cpu = smp_processor_id(); activate_task(p, this_rq()); spin_unlock(&this_rq()->lock); } else {
I was unable to figure out what the logic of the '(smp_processor_id() < p->cpu)' test is.. (Why should the CPU number of the process being awoken matter?) My best guess is that this is to enforce a locking invariant - but if so, isn't this test backwards? If p->cpu > current->cpu then p->cpu's runqueue is locked first followed by this_rq - locking greatest to least, where the rest of the code does least to greatest..
Also, this code in set_cpus_allowed() looks bogus:
if (target_cpu < smp_processor_id()) { spin_lock_irq(&target_rq->lock); spin_lock(&this_rq->lock); } else { spin_lock_irq(&target_rq->lock); spin_lock(&this_rq->lock); }
The lock order is the same regardless of the if statement..
-Kevin
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