[RFC] New Improved Stronger Whiter Timers (was: Kernel Janitor)

Daniel Phillips
2001-01-30T20:14:03Z

On Tue, 30 Jan 2001, Rusty Russell wrote:
> In message <3A74451F.DA29FD17@uow.edu.au> you write:
> > http://www.uwsg.iu.edu/hypermail/linux/kernel/0005.3/0269.html
> >
> > A lot of the timer deletion races are hard to fix because of
> > the deadlock problem.
>
> Hmmm...
>
> For 2.5, changing the timer interface to disallow mod_timer or
> add_timer (equivalent) on self, and making the timerfn return num
> jiffies to next run (0 = don't rerun) would solve this, right?
> I don't see a maintainable way of solving this otherwise,
>
> Of course, kfree'ing the timer struct and returning non-zero
> would be a *bug*...

Here's a patch that implements your idea together with a hack by me to
make it work with existing code: ->event is non-null for the new
behaviour, null for the broken behavior. The new behaviour is that
timers don't re-add themselves anymore, run_timer_list does, so when
you kill a timer it stays dead.

--- 2.4.1.clean/include/linux/timer.h Tue Jan 30 08:24:55 2001
+++ 2.4.1/include/linux/timer.h Tue Jan 30 20:59:01 2001
@@ -22,6 +22,7 @@
unsigned long expires;
unsigned long data;
void (*function)(unsigned long);
+ unsigned long (*event)(unsigned long data);
};

extern void add_timer(struct timer_list * timer);
@@ -49,6 +50,9 @@
static inline void init_timer(struct timer_list * timer)
{
timer->list.next = timer->list.prev = NULL;
+ timer->function = NULL;
+ timer->event = NULL;
+ timer->data = 0;
}

static inline int timer_pending (const struct timer_list * timer)
--- 2.4.1.clean/kernel/timer.c Sun Dec 10 18:53:19 2000
+++ 2.4.1/kernel/timer.c Tue Jan 30 20:59:01 2001
@@ -301,18 +301,25 @@
curr = head->next;
if (curr != head) {
struct timer_list *timer;
- void (*fn)(unsigned long);
- unsigned long data;
+ unsigned long data, requeue;

timer = list_entry(curr, struct timer_list, list);
- fn = timer->function;
data= timer->data;

detach_timer(timer);
timer->list.next = timer->list.prev = NULL;
timer_enter(timer);
spin_unlock_irq(&timerlist_lock);
- fn(data);
+ if (timer->event)
+ {
+ if ((requeue = timer->event(data)))
+ {
+ timer->expires += requeue;
+ internal_add_timer(timer);
+ }
+ }
+ else
+ timer->function(data); /* bad old way */
spin_lock_irq(&timerlist_lock);
timer_exit();
goto repeat;

-------------------
Here's some test code. It kprintS a 10-step countdown during init:

--- 2.4.1.clean/fs/buffer.c Mon Jan 15 21:42:32 2001
+++ 2.4.1/fs/buffer.c Tue Jan 30 20:59:01 2001
@@ -2792,9 +2792,22 @@
}
}

+struct timer_list foo_timer;
+
+unsigned long foo_event (unsigned long data)
+{
+ printk ("Foo %i\n", foo_timer.data);
+ return --foo_timer.data? HZ: 0;
+}
+
static int __init bdflush_init(void)
{
DECLARE_MUTEX_LOCKED(sem);
+ init_timer (&foo_timer);
+ foo_timer.event = foo_event;
+ foo_timer.data = 10;
+ add_timer (&foo_timer);
+
kernel_thread(bdflush, &sem, CLONE_FS | CLONE_FILES | CLONE_SIGNAL);
down(&sem);
kernel_thread(kupdate, &sem, CLONE_FS | CLONE_FILES | CLONE_SIGNAL);

--
Daniel