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Subject[PATCH] fix loop with disabled tasklets
Date

I have a questions about the current softirq implementation.

Looking at kernel/softirq.c:418 and futher. When the tasklet is disabled it
is rescheduled. This is fine, but __cpu_raise_softirq is also called. This
means that ksoftirqd will loop until the tasklet is enabled.
Normaly this is no biggy, because user processes still come through.
But during boot -when the tasklet_enable is yet to be called by "the idle
thread"- the system will lockup. This happens during the boot of my sun4m
with the keyboard tasklet.

To demonstrate, i wrote a module. Inserting this will result in 0% idle time.
Inserting this with the below patch, the system simply goes on.

If this really is an issue, this patch fixes it. It does change the current
behavior a bit though. Currently when a tasklet is run while it is already
running on a different cpu, the task is rescheduled. This results in the
tasklet being executed two times. With this patch applied, it will not. (this
is still good according to kernel/include/interrups.h:83 and futher).

i'm not sure about the enable_tasklet bit. I think it will prevent
people from calling tasklet_enable from within an interrupt handler. But then
again, why do you want to do that? Thanx, velco and

Any comments?

me

#define MODULE
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/interrupt.h>

MODULE_LICENSE("GPL");

void
dummy_lockit(unsigned long nill)
{
printk("doing the lockit\n");
return;
}
DECLARE_TASKLET_DISABLED(lockit, dummy_lockit, 0);
int
init_module(void)
{
printk("going to lock\n");
tasklet_schedule(&lockit);
return(0);
}
void
cleanup_module(void)
{
tasklet_enable(&lockit);
set_current_state(TASK_INTERRUPTIBLE);
schedule_timeout(HZ); /* some time to do dummy_lockit */
printk("bye\n");
}

diff -ruN linux-2.4.14/include/linux/interrupt.h
linux/include/linux/interrupt.h
--- linux-2.4.14/include/linux/interrupt.h Sat Nov 3 11:20:41 2001
+++ linux/include/linux/interrupt.h Sat Nov 10 12:28:14 2001
@@ -185,13 +185,15 @@
static inline void tasklet_enable(struct tasklet_struct *t)
{
smp_mb__before_atomic_dec();
- atomic_dec(&t->count);
+ if (atomic_dec_and_test(&t->count))
+ __cpu_raise_softirq(smp_processor_id(), TASKLET_SOFTIRQ);
}

static inline void tasklet_hi_enable(struct tasklet_struct *t)
{
smp_mb__before_atomic_dec();
- atomic_dec(&t->count);
+ if (atomic_dec_and_test(&t->count))
+ __cpu_raise_softirq(smp_processor_id(), HI_SOFTIRQ);
}

extern void tasklet_kill(struct tasklet_struct *t);
diff -ruN linux-2.4.14/kernel/softirq.c linux/kernel/softirq.c
--- linux-2.4.14/kernel/softirq.c Thu Nov 8 15:58:24 2001
+++ linux/kernel/softirq.c Sat Nov 10 12:27:24 2001
@@ -188,21 +188,19 @@

list = list->next;

- if (tasklet_trylock(t)) {
- if (!atomic_read(&t->count)) {
+ if (!atomic_read(&t->count)) {
+ if (tasklet_trylock(t)) {
if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
BUG();
t->func(t->data);
tasklet_unlock(t);
- continue;
}
- tasklet_unlock(t);
+ continue;
}

local_irq_disable();
t->next = tasklet_vec[cpu].list;
tasklet_vec[cpu].list = t;
- __cpu_raise_softirq(cpu, TASKLET_SOFTIRQ);
local_irq_enable();
}
}
@@ -222,21 +220,19 @@

list = list->next;

- if (tasklet_trylock(t)) {
- if (!atomic_read(&t->count)) {
+ if (!atomic_read(&t->count)) {
+ if (tasklet_trylock(t)) {
if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
BUG();
t->func(t->data);
tasklet_unlock(t);
- continue;
}
- tasklet_unlock(t);
+ continue;
}

local_irq_disable();
t->next = tasklet_hi_vec[cpu].list;
tasklet_hi_vec[cpu].list = t;
- __cpu_raise_softirq(cpu, HI_SOFTIRQ);
local_irq_enable();
}
}
-
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