Re: Recursion bug in -rt

[david singleton <dsingleton@mvista.com>]

> > The rt deadlock check is also recursive, but it stops at a depth of 
> > 20,
> > deciding something must be corrupt to have a task blocked on more
> > than 20 locks.
> >
> > We should also set a limit.  We can either just hang an ill-behaved
> > app on the waitqueue or return an error code to the application.
> > Any suggestions on which would be best, hang an illbehaved app
> > or just return it an error?
>
> Per default make the tasks block. You can always make the return code 
> an
> option on each futex, but it has to be off as default. Here is why I 
> think
> so:
>
> Having an return code would require you do the deadlock detection "up
> front" in the down() operation. That takes a lot of CPU cycles 
> .Ofcourse,
> if you return an error code, the application could use the info for 
> something
> constructive, but I am afraid must applications wont do anything 
> constructive
> about it anyway (i.e. such the application continues to run)  - such 
> error
> handling code would be hard to write.

Yes,  I agree.    I'll hang the app on a new waitqueue that will let 
the user
see with 'ps' that they are hung in futex_ill_behaved waitqueue or some 
other name
that makes it easy to see where the app is stopped.

thanks.


David

> What is needed in most application is that the stuff simply
> deadlocks with the tasks blocked on the various locks. Then you can go 
> in and
> trace the locks "postmortem".
>
> With the current setup, where you deadlock detection has to be 
> performed
> "up front" because the rt_mutex can make spinlock-deadlocks, the error
> code will be a natural thing. But when rt_mutex is "fixed" or
> replaced with something else, this feature  will force the kernel to 
> run
> deadlock detection "up front" even though it isn't used for anything
> usefull.
>
> Esben
>
> > David
> >
> > > I am working on a new way to do priority inheritance for nested locks
> > > in
> > > rt_mutex such you do not risc deadlocking the system on raw-spinlocks
> > > when you have a rt_mutex deadlock. But it wont have deadlock 
> > > detection
> > > without
> > > CONFIG_DEBUG_DEADLOCKS. On the other hand it would be possible to 
> > > make
> > > a
> > > deadlock scanner finding deadlocks in the system after they have
> > > happened.
> > > With a suitable /proc interface it could even be done in userspace.
> > >
> > > My patch to the rt_mutex is far from finished. I haven't even 
> > > compiled
> > > a
> > > kernel with it yet. I spend the little time I have between my
> > > family goes to bed and I simply have to go to bed myself writing a
> > > unittest framework for the  rt_mutex and have both the original and 
> > > the
> > > patched rt_mutex parsing all my tests. But I need more tests to 
> > > hammer
> > > out the details about task->state forinstance. If anyone is
> > > interrested I
> > > would be happy to send what I got right now.
> > >
> > > Esben
> > >
> > > > 	It's also easier to see if a POSIX compliant app has deadlocked
> > > > itself.
> > > > the 'ps' command will show that the wait channel of a deadlocked
> > > > application is waiting at 'futex_deadlock'.
> > > >
> > > > 	Let me know if it passes all your tests.
> > > >
> > > > David
> > > >
> > > >
> > > >
> > > >
> > > > On Dec 20, 2005, at 7:50 AM, Dinakar Guniguntala wrote:
> > > >
> > > > > On Tue, Dec 20, 2005 at 02:19:56PM +0100, Ingo Molnar wrote:
> > > > > > hm, i'm looking at -rf4 - these changes look fishy:
> > > > > >
> > > > > > -       _raw_spin_lock(&lock_owner(lock)->task->pi_lock);
> > > > > > +       if (current != lock_owner(lock)->task)
> > > > > > +               _raw_spin_lock(&lock_owner(lock)->task->pi_lock);
> > > > > >
> > > > > > why is this done?
> > > > >
> > > > > Ingo, this is to prevent a kernel hang due to application error.
> > > > >
> > > > > Basically when an application does a pthread_mutex_lock twice on a
> > > > > _nonrecursive_ mutex with robust/PI attributes the whole system
> > > > > hangs.
> > > > > Ofcourse the application clearly should not be doing anything like
> > > > > that, but it should not end up hanging the system either
> > > > >
> > > > > 	-Dinakar
> > > >
> > > > -
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