Re: [-rt] Fix race condition and following BUG in PI-futex

[Darren Hart <dvhltc@us.ibm.com>]

On Tuesday 01 August 2006 11:23, you wrote:
> On Tue, 2006-08-01 at 19:46 +0100, Esben Nielsen wrote:
> > I ran into the bug on 2.6.17-rt8 with the previous posted patches which
> > make pthread_timed_lock() work on UP, but the bug is there without the
> > patches - I just can't trigger it - and it is also in the mainline
> > kernel.
> >
> > The problem is that rt_mutex_next_owner() is used unprotected in
> > wake_futex_pi(). At least it isn't probably serialiazed against the next
> > owner being signalled or getting a timeout. The only lock, which is
> > good enough here, is &pi_state->pi_mutex.wait_lock, so I added this
> > protection.
> >
> > Esben
> >
> >   kernel/futex.c |   12 ++++++++++--
> >   1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > Index: linux-2.6.17-rt8/kernel/futex.c
> > ===================================================================
> > --- linux-2.6.17-rt8.orig/kernel/futex.c
> > +++ linux-2.6.17-rt8/kernel/futex.c
> > @@ -565,6 +565,7 @@ static int wake_futex_pi(u32 __user *uad
> >   	if (!pi_state)
> >   		return -EINVAL;
> >
> > +	spin_lock(&pi_state->pi_mutex.wait_lock);
> >   	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
> >
> >   	/*
> > @@ -590,15 +591,22 @@ static int wake_futex_pi(u32 __user *uad
> >   	curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
> >   	dec_preempt_count();
> >
> > -	if (curval == -EFAULT)
> > +	if (curval == -EFAULT) {
> > +		spin_unlock(&pi_state->pi_mutex.wait_lock);
> >   		return -EFAULT;
> > -	if (curval != uval)
> > +	}
> > +	if (curval != uval) {
> > +		spin_unlock(&pi_state->pi_mutex.wait_lock);
> >   		return -EINVAL;
> > +	}
> >
> >   	list_del_init(&pi_state->owner->pi_state_list);
> >   	list_add(&pi_state->list, &new_owner->pi_state_list);
> >   	pi_state->owner = new_owner;
> > +	atomic_inc(&pi_state->refcount);
>
> There really needs to be a get_pi_state() or some variant. Having to do
> a manual atomic_inc is very dangerous.

I understand the need to grab the wait_lock in order to serialize 
rt_mutex_next_owner(), but why the addition of of the atomic_inc() and the 
free_pi_state() ?  And if we do need them, shouldn't we place them around the 
use of the pi_state, rather than just before the unlock calls?

>
> > +	spin_unlock(&pi_state->pi_mutex.wait_lock);
> >   	rt_mutex_unlock(&pi_state->pi_mutex);
> > +	free_pi_state(pi_state);
>
> And to stay in line with the kernel, perhaps we should rename this to
> put_pi_state.  We aren't freeing it if there's still references to it.
>
> -- Steve
>
> >   	return 0;
> >   }
>

-- 
Darren Hart
IBM Linux Technology Center
Realtime Linux Team