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

[Steven Rostedt <rostedt@goodmis.org>]

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;
>   }