From 9f5d1c336a10c0d24e83e40b4c1b9539f7dba627 Mon Sep 17 00:00:00 2001
From: Mike Galbraith <efault@gmx.de>
Date: Wed, 4 Nov 2020 16:12:44 +0100
Subject: [PATCH] futex: Handle transient "ownerless" rtmutex state correctly
Git-commit: 9f5d1c336a10c0d24e83e40b4c1b9539f7dba627
Patch-mainline: v5.10-rc3
References: bsc#1067665
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: c1e2f0eaf015 ("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: Davidlohr Bueso <dbueso@suse.de>
---
kernel/futex.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index f8614ef4ff31..ac328874f6e5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2380,10 +2380,22 @@ 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) {