Re: [PATCH 5/6] futex: unlock before returning -EFAULT

Peter Zijlstra
2009-03-12T11:07:27Z

On Thu, 2009-03-12 at 11:47 +0100, Thomas Gleixner wrote:
> On Thu, 12 Mar 2009, Peter Zijlstra wrote:
>
> > On Thu, 2009-03-12 at 00:56 -0700, Darren Hart wrote:
> > > futex_lock_pi can potentially return -EFAULT with the rt_mutex held. This
> > > seems like the wrong thing to do as userspace should assume -EFAULT means the
> > > lock was not taken. Even if it could figure this out, we'd be leaving the
> > > pi_state->owner in an inconsistent state. This patch unlocks the rt_mutex
> > > prior to returning -EFAULT to userspace.
> >
> > lockdep would complain, one is not to leave the kernel with locks held.
>
> That would break pi futexes in bits and pieces.
>
> T1 takes F1
> T2 blocks on F1
> -> T2 sets up rt_mutex and locks it for T1
> T2 blocks on rt_mutex and boosts T1
>
> T1 calls a non futex syscall
> T1 returns from syscall with the rt_mutex still locked
>
> Thanks,

Oh right, raw rt_mutex stuff isn't lockdep annotated, and you use the
robust futex infrastructure to ensure stuff gets unlocked when holder
dies. That should work out.