Commit 6f8f2dc7 authored Nov 04, 2020 by Mike Galbraith Committed by Greg Kroah-Hartman Jan 29, 2021

UPSTREAM: futex: Handle transient "ownerless" rtmutex state correctly



commit 9f5d1c336a10c0d24e83e40b4c1b9539f7dba627 upstream.

Gratian managed to trigger the BUG_ON(!newowner) in fixup_pi_state_owner().
This is one possible chain of events leading to this:

Task Prio       Operation
T1   120	lock(F)
T2   120	lock(F)   -> blocks (top waiter)
T3   50 (RT)	lock(F)   -> boosts T1 and blocks (new top waiter)
XX   		timeout/  -> wakes T2
		signal
T1   50		unlock(F) -> wakes T3 (rtmutex->owner == NULL, waiter bit is set)
T2   120	cleanup   -> try_to_take_mutex() fails because T3 is the top waiter
     			     and the lower priority T2 cannot steal the lock.
     			  -> fixup_pi_state_owner() sees newowner == NULL -> BUG_ON()

The comment states that this is invalid and rt_mutex_real_owner() must
return a non NULL owner when the trylock failed, but in case of a queued
and woken up waiter rt_mutex_real_owner() == NULL is a valid transient
state. The higher priority waiter has simply not yet managed to take over
the rtmutex.

The BUG_ON() is therefore wrong and this is just another retry condition in
fixup_pi_state_owner().

Drop the locks, so that T3 can make progress, and then try the fixup again.

Gratian provided a great analysis, traces and a reproducer. The analysis is
to the point, but it confused the hell out of that tglx dude who had to
page in all the futex horrors again. Condensed version is above.

[ tglx: Wrote comment and changelog ]

Fixes: c1e2f0ea ("futex: Avoid violating the 10th rule of futex")
Reported-by: Gratian Crisan <gratian.crisan@ni.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/87a6w6x7bb.fsf@ni.com
Link: https://lore.kernel.org/r/87sg9pkvf7.fsf@nanos.tec.linutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 2716e78a)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I9ab0878641624448cacb1951506b5accb4c92a00

parent 4fe49963

kernel/futex.c

+14 −2

Original line number	Diff line number	Diff line
		@@ -2515,10 +2515,22 @@ static int fixup_pi_state_owner(u32 __user uaddr, struct futex_q q,
		}

		/*
		* Since we just failed the trylock; there must be an owner.
		* The trylock just failed, so either there is an owner or
		* there is a higher priority waiter than this one.
		*/
		newowner = rt_mutex_owner(&pi_state->pi_mutex);
		BUG_ON(!newowner);
		/*
		* If the higher priority waiter has not yet taken over the
		* rtmutex then newowner is NULL. We can't return here with
		* that state because it's inconsistent vs. the user space
		* state. So drop the locks and try again. It's a valid
		* situation and not any different from the other retry
		* conditions.
		*/
		if (unlikely(!newowner)) {
		err = -EAGAIN;
		goto handle_err;
		}
		} else {
		WARN_ON_ONCE(argowner != current);
		if (oldowner == current) {