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