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

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Date:	Wed, 20 Apr 2016 09:36:42 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/futex: handle the case where we got a "late"
 waiter

On Tue, 19 Apr 2016, 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.

It can be called unconditionally w/o trying the TID->0 transition in user
space first and we should handle that case.
 
Thanks,

	tglx