@@ -2380,10 +2380,22 @@ retry:
}
/*
- * 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)) {
+ ret = -EAGAIN;
+ goto handle_err;
+ }
} else {
WARN_ON_ONCE(argowner != current);
if (oldowner == current) {
The following commit has been merged into the locking/urgent branch of tip: Commit-ID: 63c1b4db662a0967dd7839a2fbaa5300e553901d Gitweb: https://git.kernel.org/tip/63c1b4db662a0967dd7839a2fbaa5300e553901d Author: Mike Galbraith <efault@gmx.de> AuthorDate: Wed, 04 Nov 2020 16:12:44 +01:00 Committer: Thomas Gleixner <tglx@linutronix.de> CommitterDate: Fri, 06 Nov 2020 22:24:58 +01:00 futex: Handle transient "ownerless" rtmutex state correctly 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 --- kernel/futex.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-)