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Date: Mon, 25 Aug 2003 05:30:16 +0530 (IST)
From: Nagendra Singh Tomar <nagendra_tomar@adaptec.com>
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To: linux-kernel@vger.kernel.org
Subject: tasklet_kill will always hang for recursive tasklets on a UP
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Hi,
	While going thru the code for tasklet_kill(), I cannot figure out 
how recursive tasklets (tasklets that schedule themselves from within 
their tasklet handler) can be killed by this function. To me it looks that 
tasklet_kill will never complete for such tasklets.

void tasklet_kill(struct tasklet_struct *t)
{
	...
 	...
	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
		current->state = TASK_RUNNING;
		do
			sys_sched_yield();
		while (test_bit(TASKLET_STATE_SCHED, &t->state));
	}
	...
	...
}

The above while loop will only exit if TASKLET_STATE_SCHED is not set 
(tasklet is not scheduled).
Now if we see tasklet_action

static void tasklet_action(struct softirq_action *a)
{
	...
	...
	if (!atomic_read(&t->count)) {
	--> TASKLET_STATE_SCHED is set here
		if(!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
			BUG();
		t->func(t->data);
	--> if we schedule the tasklet inside its handler, 
	--> TASKLET_STATE_SCHED will be set here also
		tasklet_unlock(t);
		continue;
	}
	...
	...
}

The only small window when TASKLET_STATE_SCHED is not set is between the 
time when test_and_clear_bit above clears it and by the time the tasklet 
handler again calls tasklet_schedule(). But since tasklet_kill is called 
from user context the while loop in tasklet_kill checking for 
TASKLET_STATE_SCHED to be cleared  cannot interleave between the above two 
lines in tasklet_action and hence tasklet_kill will never come out of the 
while loop.
This is true only for UP machines.

Pleae point me out if I am missing something.

Thanx
tomar