LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
* posix-timers overrun broken?
@ 2007-02-07 13:36 Milan Svoboda
  2007-02-07 16:14 ` Thomas Gleixner
  0 siblings, 1 reply; 3+ messages in thread
From: Milan Svoboda @ 2007-02-07 13:36 UTC (permalink / raw)
  To: linux-kernel

Hi,

I'm creating a driver for timer on ixp4xx (there are two hardware timers). 
It shall use posix api
so I looked at the only driver that implements it in kernel: 
drivers/char/mmtimer.c. My driver uses almost the
same logic that handles overruns.

However, I found that receiving of number of overruns in waitsiginfo is 
somewhat broken.

Linux-2.6.20:

Look at posix_timer_event - this is called when interrupt wants to signal 
the thread that
event happend. The very first operation there is
        memset(&timr->sigq->info, 0, sizeof(siginfo_t));
this clears all info members - si.overrun included.
Then send_sigqueue or send_group_sigqueue is called. They both check if 
it's
SI_TIMER and if so, they increase si.overrun if the signal is already 
queued.

But if the next interrupt arrives before function collect_signal is called 
to actually deliver the
siginfo_t to userspace, the si.overrun is cleared in posix_timer_event and 
we have just forgotten
one overrun...

Am I wrong?

Patch follows, but please note that it is more to describe the problem 
than solve the problem in proper
way...

Please, CC me, I'm not subscribed on the list.

Signed-off-by: Milan Svoboda <msvoboda@ra.rockwell.com>

Index: linux/kernel/posix-timers.c
===================================================================
--- linux.orig/kernel/posix-timers.c
+++ linux/kernel/posix-timers.c
@@ -298,7 +298,10 @@ void do_schedule_next_timer(struct sigin
 
 int posix_timer_event(struct k_itimer *timr,int si_private)
 {
+       int overrun = timr->sigq->info.si_overrun;
        memset(&timr->sigq->info, 0, sizeof(siginfo_t));
+       timr->sigq->info.si_overrun = overrun;
+
        timr->sigq->info.si_sys_private = si_private;
        /* Send signal to the process that owns this timer.*/
 
@@ -407,6 +410,7 @@ static struct k_itimer * alloc_posix_tim
                kmem_cache_free(posix_timers_cache, tmr);
                tmr = NULL;
        }
+       tmr->sigq->info.si_overrun = 0;
        return tmr;
 }
 
Index: linux/kernel/signal.c
===================================================================
--- linux.orig/kernel/signal.c
+++ linux/kernel/signal.c
@@ -402,6 +402,7 @@ static int collect_signal(int sig, struc
        if (first) {
                list_del_init(&first->list);
                copy_siginfo(info, &first->info);
+               first->info.si_overrun = 0;
                __sigqueue_free(first);
                if (!still_pending)
                        sigdelset(&list->signal, sig);


^ permalink raw reply	[flat|nested] 3+ messages in thread
* Re: posix-timers overrun broken?
@ 2007-02-08  7:55 Milan Svoboda
  0 siblings, 0 replies; 3+ messages in thread
From: Milan Svoboda @ 2007-02-08  7:55 UTC (permalink / raw)
  To: Thomas Gleixner; +Cc: linux-kernel

> On Wed, 2007-02-07 at 14:36 +0100, Milan Svoboda wrote:
> > But if the next interrupt arrives before function collect_signal is 
called 
> > to actually deliver the
> > siginfo_t to userspace, the si.overrun is cleared in posix_timer_event 
and 
> > we have just forgotten
> > one overrun...
> > 
> > Am I wrong?
> 
> Yes. posix_timer_event() is only called when the timer expires the first
> time. When the signal has been queued, the timer is not rearmed and the
> overrun is calculated in the actual signal delivery path, which calls
> do_schedule_next_timer().

Ok understand.

So it seems to me that drivers/char/mmtimer.c is wrong. It is
driver to additional hardware timers and mmtimer exports them as a posix 
timer.
When interrupt happens, they call posix_timer_event and set hardware 
interrupt
to the next tick. I use the same practise in my driver and found
that overruns are reported wrongly.

How to plug "external" source of ticks to the posix timers?

As far as I know current HRT doesn't allow us to use more sources of
ticks together, only one is selected, correct?

Milan




^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2007-02-08  7:54 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2007-02-07 13:36 posix-timers overrun broken? Milan Svoboda
2007-02-07 16:14 ` Thomas Gleixner
2007-02-08  7:55 Milan Svoboda

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).