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

[Darren Hart]

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.
>
> > 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?

Right now these are tests I know the most about and I know they 
excercise the futex_wait, futex_wake, futex_(un)lock_pi, and 
futex_requeue paths via the pthread_mutex* and pthread_cond* APIs.  I 
doubt they test the fault logic very well, and I know they don't test 
shared futexes.

I'd really like to ramp up my efforts on the raw sys_futex tests I've 
been working on, but just haven't had the cycles.  I suspect they will 
become necessary for requeue_pi however.  I also think we should look at 
some kind of futex-debug.c infrastructure that also adds some 
fault-injection to test the various error paths.

--
Darren

> > Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxx>
> > Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> > ---
> >
> >  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);
> >  
>
> --
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--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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