This is a note to let you know that I've just added the patch titled
futex: Handle transient "ownerless" rtmutex state correctly
to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
futex-handle-transient-ownerless-rtmutex-state-correctly.patch
and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From foo@baz Mon Mar 29 07:48:09 AM CEST 2021
From: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Date: Sun, 28 Mar 2021 22:43:15 +0200
Subject: futex: Handle transient "ownerless" rtmutex state correctly
To: stable@xxxxxxxxxxxxxxx
Cc: Lee Jones <lee.jones@xxxxxxxxxx>, "Luis Claudio R. Goncalves" <lgoncalv@xxxxxxxxxx>, Florian Fainelli <f.fainelli@xxxxxxxxx>
Message-ID: <YGDqY9a/qbJKK5eC@xxxxxxxxxxxxxxx>
Content-Disposition: inline
From: Mike Galbraith <efault@xxxxxx>
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: c1e2f0eaf015 ("futex: Avoid violating the 10th rule of futex")
Reported-by: Gratian Crisan <gratian.crisan@xxxxxx>
Signed-off-by: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/87a6w6x7bb.fsf@xxxxxx
Link: https://lore.kernel.org/r/87sg9pkvf7.fsf@xxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/futex.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2497,10 +2497,22 @@ 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)) {
+ err = -EAGAIN;
+ goto handle_err;
+ }
} else {
WARN_ON_ONCE(argowner != current);
if (oldowner == current) {
Patches currently in stable-queue which might be from ben@xxxxxxxxxxxxxxx are
queue-4.9/futex-drop-hb-lock-before-enqueueing-on-the-rtmutex.patch
queue-4.9/futex-rework-futex_lock_pi-to-use-rt_mutex_-_proxy_lock.patch
queue-4.9/futex-avoid-freeing-an-active-timer.patch
queue-4.9/futex-rt_mutex-introduce-rt_mutex_init_waiter.patch
queue-4.9/futex-fix-incorrect-should_fail_futex-handling.patch
queue-4.9/futex-rt_mutex-fix-rt_mutex_cleanup_proxy_lock.patch
queue-4.9/futex-use-smp_store_release-in-mark_wake_futex.patch
queue-4.9/futex-fix-possible-missed-wakeup.patch
queue-4.9/locking-futex-allow-low-level-atomic-operations-to-return-eagain.patch
queue-4.9/futex-prevent-robust-futex-exit-race.patch
queue-4.9/futex-handle-transient-ownerless-rtmutex-state-correctly.patch
queue-4.9/futex-handle-early-deadlock-return-correctly.patch
queue-4.9/arm64-futex-bound-number-of-ldxr-stxr-loops-in-futex_wake_op.patch
