Messages in this thread | | | Date | Thu, 25 Aug 2005 08:22:08 +0200 | From | Jens Axboe <> | Subject | Re: CFQ + 2.6.13-rc4-RT-V0.7.52-02 = BUG: scheduling with irqs disabled |
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On Thu, Aug 25 2005, Ingo Molnar wrote: > > * Jens Axboe <axboe@suse.de> wrote: > > > > BUG: scheduling with irqs disabled: libc6.postinst/0x20000000/13229 > > > caller is ___down_mutex+0xe9/0x1a0 > > > [<c029c1f9>] schedule+0x59/0xf0 (8) > > > [<c029ced9>] ___down_mutex+0xe9/0x1a0 (28) > > > [<c0221832>] cfq_exit_single_io_context+0x22/0xa0 (84) > > > [<c02218ea>] cfq_exit_io_context+0x3a/0x50 (16) > > > [<c021db84>] exit_io_context+0x64/0x70 (16) > > > [<c011efda>] do_exit+0x5a/0x3e0 (20) > > > [<c011f3ca>] do_group_exit+0x2a/0xb0 (24) > > > [<c0103039>] syscall_call+0x7/0xb (20) > > > > Hmm, Ingo I seem to remember you saying that the following construct: > > > > local_irq_save(flags); > > spin_lock(lock); > > > > which is equivelant to spin_lock_irqsave() in mainline being illegal > > in -RT, is that correct? This is what cfq uses right now for an > > exiting task, as the above trace indicates. > > yes, that's correct. Mainline's exit_io_contexts() uses the above > construct because there's no task_lock_irqsave(current, flags) API. > > note that recent -RT kernels are a lot less drastic about these cases > and print a once-per-bootup warning, not a scary message like above. > This message should not happen in recent -RT kernels. > > The problem was this piece of code in exit_io_contexts(): > > local_irq_save(flags); > task_lock(current); > ioc = current->io_context; > current->io_context = NULL; > ioc->task = NULL; > task_unlock(current); > local_irq_restore(flags); > > i understand the detached use of flags, but i'm also wondering why irqs > have to be disabled here in the first place? At this point in do_exit() > we should normally not have any pending IO attached to our io_context. > What am i missing?
There can quite easily be lots of pending IO for the io_context (and, in CFQ's case, below cfq_io_contexts), task exiting is completely decoupled from any pending io.
Then there's the cfq_exit_io_context() locking. I have to ponder this a bit, I cannot even convince myself that it is currently safe right now.
-- Jens Axboe
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