Re: route cache DoS testing and softirqs

From: Srivatsa Vaddagiri
Date: Tue Mar 30 2004 - 00:09:25 EST

On Tue, Mar 30, 2004 at 01:07:12AM +0000, Andrea Arcangeli wrote:
> btw, the set_current_state(TASK_INTERRUPTIBLE) before
> kthread_should_stop seems overkill w.r.t. smp locking, plus the code is
> written in the wrong way around, all set_current_state are in the wrong
> place. It's harmless but I cleaned up that bit as well.

I think set_current_state(TASK_INTERRUPTIBLE) before kthread_should_stop()
_is_ required, otherwise kthread_stop can fail to destroy a kthread.

kthread_stop does:

1. kthread_stop_info.k = k;
2. wake_up_process(k);

and if ksoftirqd were to do :

a. while (!kthread_should_stop()) {
b. __set_current_state(TASK_INTERRUPTIBLE);
c. schedule();

There is a (narrow) possibility here that a) happens _after_ 1) as well as
b) _after_ 2).

In such a case kthread_stop would have failed to wake up the kthread!

The same race is avoided if ksoftirqd does:

a. __set_current_state(TASK_INTERRUPTIBLE);
b. while (!kthread_should_stop()) {
c. schedule();
d. __set_current_state(TASK_INTERRUPTIBLE);

e. __set_current_state(TASK_RUNNING);

In this case, even if b) happens _after_ 1) and c) _after_ 2), schedule
simply returns immediately because task's state would have been set to
TASK_RUNNING by 2). It goes back to the kthread_should_stop() check and


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017
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