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

Peter Zijlstra
2009-03-12T10:14:12Z

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.

> Build and boot tested on a 4 way Intel x86_64 workstation. Passes basic
> pthread_mutex and PI tests out of ltp/testcases/realtime.

You keep mentioning these tests.. makes me wonder how much of the futex
code paths they actually test. Got any coverage info on them?

> Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Rusty Russell <rusty@rustcorp.com.au>
> ---
>
> kernel/futex.c | 7 +++++++
> 1 files changed, 7 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 6579912..c980a55 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1567,6 +1567,13 @@ retry_locked:
> }
> }
>
> + /*
> + * If fixup_pi_state_owner() faulted and was unable to handle the
> + * fault, unlock it and return the fault to userspace.
> + */
> + if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current))
> + rt_mutex_unlock(&q.pi_state->pi_mutex);
> +
> /* Unqueue and drop the lock */
> unqueue_me_pi(&q);
>
>