Messages in this thread | | | From | Darren Hart <> | Subject | Re: [-rt] Fix race condition and following BUG in PI-futex | Date | Tue, 1 Aug 2006 13:22:52 -0700 |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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