linux-kernel - Re: [PATCH] kernel/futex: handle the case where we got a "late" waiter

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Date:	Wed, 20 Apr 2016 09:09:46 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/futex: handle the case where we got a "late"
 waiter

On 04/20/2016 12:27 AM, Davidlohr Bueso wrote:
> On Fri, 15 Apr 2016, Sebastian Andrzej Siewior wrote:
> 
>> futex_unlock_pi() gets uval before taking the hb lock. Now imagine
>> someone in futex_lock_pi() took the lock. While futex_unlock_pi() waits
>> for the hb lock, the LOCK_PI sets FUTEX_WAITERS and drops the lock.
>> Now, futex_unlock_pi() figures out that there is waiter and invokes
>> wake_futex_pi() with the old uval which does not yet have FUTEX_WAITERS
>> set. This flaw lets cmpxchg_futex_value_locked() fail and return -EINVAL.
> 
> Hmm but if we're calling futex_unlock_pi() in the first place, doesn't that
> indicate that the uval already has FUTEX_WAITERS and therefore failed the
> TID->0 transition in userland? That or the thread is bogusly unlocking a
> lock that it doesn't own.

I hacked a testing tool which always did the syscall for LOCK_PI /
UNLOCK_PI and this popped up.

> 
> This is of course different than the requeue_pi case which can specify
> set_waiters but also gets the value via get_futex_value_locked().
> 
> Is this a real issue or did you find it by code inspection?

real issue.
	https://breakpoint.cc/mass-futex2-rl.c

> Thanks,
> Davidlohr

Sebastian