Messages in this thread | | | Date | Tue, 30 Jan 2001 18:49:32 +0100 | From | Daniel Phillips <> |
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Daniel Phillips wrote: > > 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, > > It seems silly not to provide direct support for such a simple, useful > mechanism as a periodic timer. This can be accomplished easily by > adding a field 'periodic' to struct timer_list. If 'periodic' is > non-zero then run_timer_list uses it to set the 'expires' field and > re-inserts the timer. > > For what it's worth, this is backward compatible with the existing > strategy. The timer_list->function is still in complete control of > things if it wants to be, but forbidding it from re-adding itself sounds > like an awfully good idea.
Whoops, this post from Alexy makes it quite clear why I can't do that:
http://www.wcug.wwu.edu/lists/netdev/200005/msg00050.html Timers are self-destructable as rule. See? Normal usage for timer is to have it allocated inside an object and timer event detroys the object together with timer.
I did a quick scan through timer usage, and sure enough, I found self-destructive behaviour as Alexy describes, for example, in ax25_std_heartbeat_expiry. Your suggestion is good and simple, but requires every timer_list->function to be changed, a couple of hundred places to check.
It would be nice to have a nice easy transition instead of a jump-off-the-cliff and change all usage approach. Hmm, a hack is coming... I'll add a new, improved function field beside the old one, call it ->timer_event, and it can force rescheduling as you suggested. If ->timer_event is non-null it gets called instead of ->function, and the timer may be requeued. For good measure, I'll leave my ->period field in there because it just makes sense. Then I can write a generic ->timer_event that just returns the ->period.
/me: hack, hack, hack
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