Re: CFQ + 2.6.13-rc4-RT-V0.7.52-02 = BUG: scheduling with irqs disabled

Ingo Molnar
2005-08-25T06:09:14Z

* 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?

Ingo