futex: Handle unlock_pi race gracefully
authorSebastian Andrzej Siewior <bigeasy@linutronix.de>
Fri, 15 Apr 2016 12:35:39 +0000 (14:35 +0200)
committerThomas Gleixner <tglx@linutronix.de>
Wed, 20 Apr 2016 10:33:13 +0000 (12:33 +0200)
commit89e9e66ba1b3bde9d8ea90566c2aee20697ad681
tree9cd0c1ecd75e6bbf03588e0c6d9b20e3d811c86f
parent6687659568e2ec5b3ac24b39c5d26ce8b9d90434
futex: Handle unlock_pi race gracefully

If userspace calls UNLOCK_PI unconditionally without trying the TID -> 0
transition in user space first then the user space value might not have the
waiters bit set. This opens the following race:

CPU0                 CPU1
uval = get_user(futex)
    lock(hb)
lock(hb)
    futex |= FUTEX_WAITERS
    ....
    unlock(hb)

cmpxchg(futex, uval, newval)

So the cmpxchg fails and returns -EINVAL to user space, which is wrong because
the futex value is valid.

To handle this (yes, yet another) corner case gracefully, check for a flag
change and retry.

[ tglx: Massaged changelog and slightly reworked implementation ]

Fixes: ccf9e6a80d9e ("futex: Make unlock_pi more robust")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1460723739-5195-1-git-send-email-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
kernel/futex.c