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(-)

diff --git a/kernel/futex.c b/kernel/futex.c index f8614ef..7406914 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -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) {