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

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Message-ID: <87im5qzxp5.fsf@nanos.tec.linutronix.de>
Date:   Tue, 16 Mar 2021 20:48:54 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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, 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?)
>
> However, as 34b1a1ce145 describes, it was cared enough about users to
> protect them against spurious runaway tasks. And this is why I decided
> to even send the patch; it fixes, without sacrificing performance or
> additional complexity, a potentially user visible issue which could be
> due to programming error. And unlike 34b1a1ce145, where a stealer that
> cannot fault ends up dropping the lock, here the stealer can actually
> amend things and not break semantics because of another task's stupidity.
> But yeah, this could also be considered in the category of inept attempts
> to fix a rotten situation.

It's one of the 'Doctor it hurts when I shoot myself in the foot' cases :)