Ingo Molnar <[email protected]> wrote:
>
>
> * Steven Rostedt <[email protected]> wrote:
>
> > > proc_subdir_lock can also be used from softirq (tasklet) context, which
> > > may lead to deadlocks.
> > >
> > > This bug was found via the lock validator:
> > >
> >
> > Thanks Ingo,
> >
> > I stressed in sending the patch that there was a big assumption that
> > the calls would not be done in (soft)irq context. I just didn't want
> > to add overhead if it wasn't needed. But I guess that this is needed
> > until we can remove all the instances that use it in softirq context.
> > But that's for a later patch.
>
> the validator just found another problem with this lock, pointing out
> that files_lock nests inside of proc_subdir_lock, and that files_lock is
> a softirq-unsafe lock, creating another (unlikely but possible) deadlock
> scenario:
files_lock can be taken on the free_irq() path: proc_kill_inodes().
> ...
> to solve this we must either change files_lock to be softirq-safe too
> (bleh!), or we must forbid remove_proc_entry() use from softirq
> contexts. Neither is a happy solution - remove_proc_entry() is used
> within free_irq(), and who knows how many drivers do free_irq() in
> softirq/tasklet context ...
free_irq()'s /proc fiddling has always been a pain - we just shouldn't be
doing filesystem things in irq/bh context.
> Andrew, this needs to be resolved before v2.6.16, correct? Steve's patch
> solves a real bug in the upstream kernel.
It's not a very big bug - I think only Steve hit it, and that with a
stress-test which was somewhat tuned to hit it.
So we can afford to sit on the problem for a while, as long as someone is
working on a broader /proc-sanity fix. But nobody will do that.
I wonder if we can just punt the unregister_handler_proc/kfree up to a
keventd callback.
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