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

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Date:	Tue, 19 Apr 2016 15:27:37 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 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.

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?

Thanks,
Davidlohr