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

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Message-ID: <alpine.LFD.2.00.0903121142200.29264@localhost.localdomain>
Date:	Thu, 12 Mar 2009 11:47:56 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
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, 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,

	tglx
   
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