Re: Recursion bug in -rt

[Steven Rostedt]

On Tue, 2005-12-20 at 21:42 +0100, Esben Nielsen wrote:

> > 
> This is ok for kernel mutexes, which are supposed not to cause deadlocks.
> But user space deadlocks must not cause kernel deadlocks. Therefore the
> robust futex code _must_ be fixed.

And it should be fixed in the futex side. Not in rt.c.

> 
> > The benefit of this locking order is that we got rid of the global
> > pi_lock, and that was worth the problems you face today.
> >
> 
> I believe this problem can be solved in the pi_lock code - but it will
> require quite a bit of recoding. I started on it a little while ago but
> didn't at all get the time to get into anything to even compile :-(
> I don't have time to finish any code at all but I guess I can plant an
> the idea instead:

I removed the global pi_lock in two sleepless days, and it was all on
the theory that the locks themselves would not deadlock.  That was the
only sanity I was able to hang on to.  That was complex enough, and very
scary to get right (scary since it _had_ to be done right).  And it was
complex enough to keep a highly skilled programmer living in Hungary
from doing it himself (not to say he couldn't do it, just complex enough
for him to put it off for quite some time).  And one must also worry
about the BKL which is a separate beast all together.

So making it any more complex is IMO out of the question.

> 
> When resolving the mutex chain (task A locks mutex 1 owned by B blocked
> on 2 owned by C etc) for PI boosting and also when finding deadlocks,
> release _all_ locks before going to the next step in the chain. Use
> get_task_struct on the next task in the chain, release the locks,
> take the pi_locks in a fixed order (sort by address forinstance), do the
> PI boosting, get_task_struct on the next lock, release all the locks, do
> the put_task_struct() on the previous task etc.
> This a lot more expensive approach as it involves double as many spinlock
> operations and get/put_task_struct() calls , but it has the added benifit
> of reducing the overall system latency for long locking chains as there
> are spots where interrupts and preemption will be enabled for each step in
> the chain. Remember also that deep locking chains should be considered an
> exeption so the code doesn't have to be optimal wrt. performance. 

The long lock holding is only by the lock being grabbed and the owner
grabbing it.  All other locks don't need to be held for long periods of
time.  There's lots of issues if you release these two locks. How do you
deal with the mutex being released while going up the chain?

> 
> I added my feeble attempts to implement this below. I have no chance of
> ever getting time finish it :-(

I doubt they were feeble, but just proof that this approach is far too
complex.  As I said, if you don't want futex to deadlock the kernel, the
API for futex should have deadlocking checks, since the only way this
can deadlock the system, is if two threads are in the kernel at the same
time.

Also, the ones who are paying me to help out the -rt kernel, don't care
if we let the user side deadlock the system.  They deliver a complete
package, kernel and user apps, and nothing else is to be running on the
system.  This means that if the user app deadlocks, it doesn't matter if
the kernel deadlocks or not, because the deadlocking of the user app
means the system has failed.  And I have a feeling that a lot of other
users of -rt feel the same way.

-- Steve


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