linux-kernel - Re: [patch V2 5/5] futex: Simplify futex_lock_pi

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Date:	Tue, 17 Jun 2014 09:20:29 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Darren Hart <darren@...art.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Kees Cook <kees@...flux.net>, wad@...omium.org
Subject: Re: [patch V2 5/5] futex: Simplify futex_lock_pi_atomic() and make
 it more robust

On Mon, 16 Jun 2014, Darren Hart wrote:
> On Fri, 2014-06-13 at 11:44 +0200, Thomas Gleixner wrote:
> Two general concerns, we appear to be eliminating both the force_take
> and the retry.
> 
> The force_take only occurs if TID==0, and that is covered here in a
> cleaner way, so I believe we are good here.
> 
> As for the retry, the remaining use case (outside of TID==0 ->
> force_take=1 -> retry) appears to be that userspace changed the value
> while we were running. Reading the value early doesn't protect us from
> this scenario. How does this change account for that?
>       
> It looks to me that before we would retry, while here we just give up
> and return -EAGAIN..., which is addressed in futex_lock_pi(), but not in
> the futex_requeue() callsite for futex_proxy_trylock_atomic. It does
> handle it, but I guess also needs a comment update to "The owner was
> exiting" to include "or userspace changed the value" as you did for
> futex_lock_pi().

Right. I moved the handling back to the call site which has to handle
the various error return codes anyway. 
 
> >From my analysis, this is a good cleanup and makes the code for
> explicit. I'm nervous about missing corner cases, and would like to
> understand what level of testing this has received. We need to add PI
> locking tests to futextest. There are some in glibc. Which tests were
> run to validate PI locking?

I ran it against everything I have. libc tests, your stuff, rt-tests
and random exploit and corner case exposure code I collected/wrote in
the last few weeks.

Thanks,

	tglx
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