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

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Message-Id: <1236856017.5090.235.camel@laptop>
Date:	Thu, 12 Mar 2009 12:06:57 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Darren Hart <dvhltc@...ibm.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 5/6] futex: unlock before returning -EFAULT

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.

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