Re: robust futex deadlock detection patch

[David Singleton <dsingleton@mvista.com>]

Esben Nielsen wrote:

> I am a little bit confused when I read check_futex_deadlock():
> It takes a parameter struct thread_info *ti and immediately do
> struct task_struct *task = ti->task. Now we have the usual pair
> (thread_info *ti, task_t *task) corresponding to the same process. Later
> on in the function you do ti = lock_owner(lock), but do not update task.
> Was this intented?
>  

Whoops.  You are right.  I've fixed the update to the task structure.  
The check_futex_deadlock()
code now mirrors the existing check_deadlock() code.

> Anyway, I can't see that you have locked the necesary raw_spin_locks.
> Forinstance lock_owner(lock) must be called with the lock->wait_lock taken
> and task->blocked_on needs task->pi_lock locked.
>  

Actually those locks are grabbed in the down_try_futex code.  I hold 
both of those
locks across the check for deadlocks and into __down_interruptible.   
Those locks
need to be held for the check for deadlocks and holding
them from down_try_futex to down_interruptible garuantees that a thread
that enters the kernel to block on a lock will block on the lock.  There 
is no
window any more between dropping the robust_sem and mmap_sem
and calling down_interruptible.

This also fixed an SMP problem that Dave Carlson has been seeing on earlier
patches.

The new patch is at

   http://source.mvista.com/~dsingleton/patch-2.6.15-rt2-rf2

David

> To avoid deadlocks in all the deadlock detection you have to do the loop
> something like
>
> for(owner = current; owner; ) {
> raw_spin_lock(&owner->pi_lock);
> if(owner->task->blocked_on) {
>   lock = owner->task->blocked_on->lock;
>   raw_spin_lock(&lock->wait_lock);
>   owner2 = lock_owner(lock);
>   if(owner2) {
>     get_task_struct(owner2->task);
>     raw_spin_unlock(&lock->wait_lock)
>     raw_spin_unlock(&owner->pi_lock);
>   }
> put_task_struct(owner->task);
> owner = owner2;
> if(owner2==current) DEADLOCK
> }
>
>
> Esben
>
>
>  
>
> > David
> >
> > -
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> >    
>
>
>
>  

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