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

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

[Esben Nielsen]

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);
+	spin_unlock(&pi_state->pi_mutex.wait_lock);
  	rt_mutex_unlock(&pi_state->pi_mutex);
+	free_pi_state(pi_state);

  	return 0;
  }

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

[Steven Rostedt]

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.

> +	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;
>   }

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

[Darren Hart]

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

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

[Steven Rostedt]

On Tue, 2006-08-01 at 13:22 -0700, Darren Hart wrote:

> > >
> > >   	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?

Hmm, is the inc really needed?  The hb->lock is held through this and
the pi_state can't go away while that lock is held.

-- Steve

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

[Esben Nielsen]

On Tue, 1 Aug 2006, Steven Rostedt wrote:

> On Tue, 2006-08-01 at 13:22 -0700, Darren Hart wrote:
>
>>>>
>>>>   	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?
>
> Hmm, is the inc really needed?  The hb->lock is held through this and
> the pi_state can't go away while that lock is held.

I was going to ask about that... If you say so they can go. I just added 
the inc/dec to be sure.

Esben

>
> -- Steve
>
>

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

[Darren Hart]

On Tuesday 01 August 2006 16:28, you wrote:
> On Tue, 1 Aug 2006, Steven Rostedt wrote:
> > On Tue, 2006-08-01 at 13:22 -0700, Darren Hart wrote:
> >>>>   	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?
> >
> > Hmm, is the inc really needed?  The hb->lock is held through this and
> > the pi_state can't go away while that lock is held.
>
> I was going to ask about that... If you say so they can go. I just added
> the inc/dec to be sure.

So the only thing that frees the pi_state is free_pi_state and it's only 
caller is unqueue_me_pi() which must be called with hb->lock held, so since
we already hold it, I think we're fine without the inc/free lines (as Steven
already said).  The following patch has the lines removed.

The direct use of the rt_mutex wait_lock seems a little out of place here, as
it "ought to be" restricted to rt_mutex.c.  Perhaps some kind of an "atomic"
rt_mutex_set_next_owner() call could abstract this away from futex.c?  I 
confess I don't see a way to do that without putting futex internals into
rt_mutex.c... so not really any better.


Index: 2.6.17-rt8/kernel/futex.c
===================================================================
--- 2.6.17-rt8.orig/kernel/futex.c	2006-08-02 20:29:42.000000000 -0700
+++ 2.6.17-rt8/kernel/futex.c	2006-08-02 21:39:17.000000000 -0700
@@ -565,6 +565,7 @@
 	if (!pi_state)
 		return -EINVAL;
 
+	spin_lock(&pi_state->pi_mutex.wait_lock);
 	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
 
 	/*
@@ -598,6 +599,8 @@
 	list_del_init(&pi_state->owner->pi_state_list);
 	list_add(&pi_state->list, &new_owner->pi_state_list);
 	pi_state->owner = new_owner;
+
+	spin_unlock(&pi_state->pi_mutex.wait_lock);
 	rt_mutex_unlock(&pi_state->pi_mutex);
 
 	return 0;



-- 
Darren Hart
IBM Linux Technology Center
Realtime Linux Team