linux-kernel - Re: [PATCH 2/2] futex: Leave the pi lock stealer in a consistent state upon successful fault

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Date:   Tue, 16 Mar 2021 21:12:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     tglx@...utronix.de, mingo@...hat.com, dvhart@...radead.org,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/2] futex: Leave the pi lock stealer in a consistent
 state upon successful fault

On Tue, Mar 16, 2021 at 11:03:05AM -0700, Davidlohr Bueso wrote:
> On Tue, 16 Mar 2021, Peter Zijlstra wrote:
> > 
> > IIRC we made the explicit choice to never loop here. That saves having
> > to worry about getting stuck in in-kernel loops.
> > 
> > Userspace triggering the case where the futex goes corrupt is UB, after
> > that we have no obligation for anything to still work. It's on them,
> > they get to deal with the bits remaining.
> 
> I was kind of expecting this answer, honestly. After all, we are warned
> about violations to the 10th:
> 
>  * [10] There is no transient state which leaves owner and user space
>  *      TID out of sync. Except one error case where the kernel is denied
>  *      write access to the user address, see fixup_pi_state_owner().
> 
> (btw, should we actually WARN_ON_ONCE this case such that the user is
> well aware things are screwed up?)

I'm not sure WARN is appropriate, it is something unpriv userspace
can trigger at will.