Pieter has found this bug in -rt:
We are experiencing 'soft' deadlocks when running our code (Freebob,
i.e. userspace lib for firewire audio) on RT kernels. After a few
seconds of system freeze, I get a kernel panic message that signals a soft lockup.
I've uploaded the photo's of the panic here:
http://freebob.sourceforge.net/old/img_3378.jpg (without flash)
http://freebob.sourceforge.net/old/img_3377.jpg (with flash)
both are of suboptimal quality unfortunately, but all info is readable
on one or the other.
The problems occur when an ISO stream (receive and/or transmit) is shut
down in a SCHED_FIFO thread. More precisely when running the freebob
jackd backend in real-time mode. And even more precise: they only seem
to occur when jackd is shut down. There are no problems when jackd is
started without RT scheduling.
I havent been able to reproduce this with other test programs that are
shutting down streams in a SCHED_FIFO thread.
The problem is not reproducible on non-RT kernels, and it only occurs on those configured for
PREEMPT_RT. If I use PREEMPT_DESKTOP, there is no problem. The PREEMPT_DESKTOP setting was the only change between the two tests, all other kernel settings (threaded irq's etc...) were unchanged.
My tests are performed on 2.6.17-rt1, but the lockups are confirmed for
PREEMPT_RT configured kernels 2.6.14 and 2.6.16.
Some extra information:
Lee Revell wrote:
> <...>
>
> It seems that the -rt patch changes tasklet_kill:
>
> Unpatched 2.6.17:
>
> void tasklet_kill(struct tasklet_struct *t)
> {
> if (in_interrupt())
> printk("Attempt to kill tasklet from interrupt\n");
>
> while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
> do
> yield();
> while (test_bit(TASKLET_STATE_SCHED, &t->state));
> }
> tasklet_unlock_wait(t);
> clear_bit(TASKLET_STATE_SCHED, &t->state);
> }
>
> 2.6.17-rt:
>
> void tasklet_kill(struct tasklet_struct *t)
> {
> if (in_interrupt())
> printk("Attempt to kill tasklet from interrupt\n");
>
> while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
> do msleep(1);
> while (test_bit(TASKLET_STATE_SCHED, &t->state));
> }
> tasklet_unlock_wait(t);
> clear_bit(TASKLET_STATE_SCHED, &t->state);
> }
>
> You should ask Ingo & the other -rt developers what the intent of this
> change was. Obviously it loops forever waiting for the state bit to
> change.
On Thu, 2006-07-06 at 22:14 +0200, Pieter Palmers wrote:
> > I've put the debugging printk's into tasklet_kill. One interesting
> > remark is that now that they are in place, I had to start/stop jackd
> > multiple times before deadlock occurs. Without the printk's the machine
> > always locked up on the first pass. However I stopped after the first
> > lockup, so maybe this is not really significant.
> >
> > Anyway, the new tasklet_kill looks like this:
> >
> > void tasklet_kill(struct tasklet_struct *t)
> > {
> > printk("enter tasklet_kill\n");
> > if (in_interrupt())
> > printk("Attempt to kill tasklet from interrupt\n");
> >
> > printk("passed interrupt check\n");
> >
> > while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
> > do
> > msleep(1);
> > while (test_bit(TASKLET_STATE_SCHED, &t->state));
> > }
> > printk("passed test_and_set_bit\n");
> >
> > tasklet_unlock_wait(t);
> > printk("passed tasklet_unlock_wait\n");
> >
> > clear_bit(TASKLET_STATE_SCHED, &t->state);
> > }
> >
> > And the last line printed before lockup is:
> > "passed test_and_set_bit"
>
This makes the change in tasklet_unlock_wait() as the prime suspect for this problem.
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