Re: [PATCH rc4-mm2 2/2] posix-timers: use try_to_del_timer_sync()

[George Anzinger <george@mvista.com>]

Oleg Nesterov wrote:
> George Anzinger wrote:
>
> > Oleg Nesterov wrote:
> >
> > > This patch removes timer_active/set_timer_inactive which plays with
> > > timer_list's internals in favour of using try_to_del_timer_sync(),
> > > which was introduced in the previous patch.
> >
> > Is there a particular reason for this, like it does not work, for example, or
> > are you just trying to clean up code?
>
>
> It's a cleanup, I think that current code is correct.
>
>
> > If this currently works, please leave it alone.
>
>
> Ok.
>
>
> > We also note that this code is the subject of a patch to the RT patch to cover
> > the same issue when softirqs are run from threads and therefor allow
> > posix_timer_fn to be preempted.  (That fix being mainly to expand usage from
> > just SMP to SMP || SOFTIRQ_PREEMPT.)
>
>
> I guess you are talking about this patch:
> http://marc.theaimsgroup.com/?l=linux-kernel&m=111566867218576
>
>
> > Also, I think that del_timer_sync and friends need to be turned on if soft_irq
> > is preemptable.
>
>
> I agree completely.
>
>
> > + * For RT the timer call backs are preemptable.  This means that folks
> > + * trying to delete timers may run into timers that are "active" for
> > + * long times.  To help out with this we provide a wake up function to
> > + * wake up a caller who wants waking when a timer clears the call back.
> > + * This is the same sort of thing that the del_timer_sync does, but we
> > + * need (in the HRT case) to cover two lists and not just the one.
> > + */
> > +#ifdef CONFIG_PREEMPT_SOFTIRQS
> > +#include <linux/wait.h>
> > +static DECLARE_WAIT_QUEUE_HEAD(timer_wake_queue);
> > +#define wake_timer_waiters() wake_up(&timer_wake_queue)
> > +#define wait_for_timer(timer) wait_event(timer_wake_queue, !timer_active(timer))
>
>
> I'm not an expert at all, so I may be wrong, but I don't think
> it's a good idea.
>
> I think it is bad if __run_timers() could be preempted while
> ->running_timer != NULL. This will interact badly with __mod_timer,
> del_timer_sync. I think that __run_timers() should do:
>
> 	set_running_timer(base, timer);
> 	preempt_disable();
> 	spin_unlock_irq(&base->lock);
>
> 	timer->function();
>
> 	set_running_timer(base, NULL);
> 	preempt_enable();
> 	spin_lock_irq(&base->lock);
>
> What do you think?

First, I think we need to get Ingo in the discussion. :)

Second, the RT patch has been running this way with little problems, save a 
REALLY intense test we (Monta Vista) have run that, from time to time, shows 
this to be a problem in the posix-timer code that is fixed by including 
SOFTIRQ_PREEMPT as well as SMP in the timer ifdefs.

One thing I do see there (in the RT patch) is a change to del_timer_sync to wait 
for the timer call back to complete rather than to loop...
>
--
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/
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