futex: Prevent lockup in requeue-PI during signal/ timeout wakeup

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[ Upstream commit bc7304f3ae ]

During wait-requeue-pi (task A) and requeue-PI (task B) the following
race can happen:

     Task A                             Task B
  futex_wait_requeue_pi()
    futex_setup_timer()
    futex_do_wait()
                                   futex_requeue()
                                        CLASS(hb, hb1)(&key1);
                                        CLASS(hb, hb2)(&key2);
        *timeout*
    futex_requeue_pi_wakeup_sync()
        requeue_state = Q_REQUEUE_PI_IGNORE

    *blocks on hb->lock*

                                        futex_proxy_trylock_atomic()
                                          futex_requeue_pi_prepare()
                                            Q_REQUEUE_PI_IGNORE => -EAGAIN
                                        double_unlock_hb(hb1, hb2)
                                         *retry*

Task B acquires both hb locks and attempts to acquire the PI-lock of the
top most waiter (task B). Task A is leaving early due to a signal/
timeout and started removing itself from the queue. It updates its
requeue_state but can not remove it from the list because this requires
the hb lock which is owned by task B.

Usually task A is able to swoop the lock after task B unlocked it.
However if task B is of higher priority then task A may not be able to
wake up in time and acquire the lock before task B gets it again.
Especially on a UP system where A is never scheduled.

As a result task A blocks on the lock and task B busy loops, trying to
make progress but live locks the system instead. Tragic.

This can be fixed by removing the top most waiter from the list in this
case. This allows task B to grab the next top waiter (if any) in the
next iteration and make progress.

Remove the top most waiter if futex_requeue_pi_prepare() fails.
Let the waiter conditionally remove itself from the list in
handle_early_requeue_pi_wakeup().

Fixes: 07d91ef510 ("futex: Prevent requeue_pi() lock nesting issue on RT")
Reported-by: Moritz Klammler <Moritz.Klammler@ferchau.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260428103425.dywXyPd3@linutronix.de
Closes: https://lore.kernel.org/all/VE1PR06MB6894BE61C173D802365BE19DFF4CA@VE1PR06MB6894.eurprd06.prod.outlook.com
[cmllamas: adapt to 5.15, apply hunk to kernel/futex/core.c prior to the
 requeue code split and use hb_waiters_dec() before the helper rename]
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

...

This commit is contained in:

Sebastian Andrzej Siewior 2026-07-22 20:47:48 +00:00 • committed by Greg Kroah-Hartman

parent 7e558308eb

commit e5dbae3572

1 changed files with 9 additions and 4 deletions

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kernel/futex/core.c

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