Messages in this thread | | | Date | Sun, 21 Jan 2007 20:30:45 -0300 | From | Pieter Palmers <> | Subject | status of: tasklet_unlock_wait() causes soft lockup with -rt and ieee1394 audio |
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Dear all,
What is the status with respect to this problem? I see that in the current -rt patch the problematic code piece is different. I personally haven't tried to reproduce this myself on a more recent kernel, but I just got a report from one of our users who experienced the same problem with 2.6.19-rt15 and RT preemption (desktop preemption works fine).
Should the latest -rt patches be fixed with respect to this issue? If so I'll try and test them, otherwise I omit the effort.
Thanks,
Pieter
Lee Revell wrote: > 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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