This is a note to let you know that I've just added the patch titled futex: Futex_unlock_pi() determinism 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-futex_unlock_pi-determinism.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 Thu Mar 4 02:09:29 PM CET 2021 From: Ben Hutchings <ben@xxxxxxxxxxxxxxx> Date: Mon, 1 Mar 2021 18:31:39 +0100 Subject: futex: Futex_unlock_pi() determinism To: stable@xxxxxxxxxxxxxxx Cc: Lee Jones <lee.jones@xxxxxxxxxx>, "Luis Claudio R. Goncalves" <lgoncalv@xxxxxxxxxx> Message-ID: <YD0k+9Ukc5MhhU8V@xxxxxxxxxxxxxxx> Content-Disposition: inline From: Ben Hutchings <ben@xxxxxxxxxxxxxxx> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> commit bebe5b514345f09be2c15e414d076b02ecb9cce8 upstream. The problem with returning -EAGAIN when the waiter state mismatches is that it becomes very hard to proof a bounded execution time on the operation. And seeing that this is a RT operation, this is somewhat important. While in practise; given the previous patch; it will be very unlikely to ever really take more than one or two rounds, proving so becomes rather hard. However, now that modifying wait_list is done while holding both hb->lock and wait_lock, the scenario can be avoided entirely by acquiring wait_lock while still holding hb-lock. Doing a hand-over, without leaving a hole. Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: juri.lelli@xxxxxxx Cc: bigeasy@xxxxxxxxxxxxx Cc: xlpang@xxxxxxxxxx Cc: rostedt@xxxxxxxxxxx Cc: mathieu.desnoyers@xxxxxxxxxxxx Cc: jdesfossez@xxxxxxxxxxxx Cc: dvhart@xxxxxxxxxxxxx Cc: bristot@xxxxxxxxxx Link: http://lkml.kernel.org/r/20170322104152.112378812@xxxxxxxxxxxxx Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/futex.c | 24 +++++++++++------------- 1 file changed, 11 insertions(+), 13 deletions(-) --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1555,15 +1555,10 @@ static int wake_futex_pi(u32 __user *uad WAKE_Q(wake_q); int ret = 0; - raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); - if (!new_owner) { + if (WARN_ON_ONCE(!new_owner)) { /* - * Since we held neither hb->lock nor wait_lock when coming - * into this function, we could have raced with futex_lock_pi() - * such that we might observe @this futex_q waiter, but the - * rt_mutex's wait_list can be empty (either still, or again, - * depending on which side we land). + * As per the comment in futex_unlock_pi() this should not happen. * * When this happens, give up our locks and try again, giving * the futex_lock_pi() instance time to complete, either by @@ -3018,15 +3013,18 @@ retry: if (pi_state->owner != current) goto out_unlock; + get_pi_state(pi_state); /* - * Grab a reference on the pi_state and drop hb->lock. + * Since modifying the wait_list is done while holding both + * hb->lock and wait_lock, holding either is sufficient to + * observe it. * - * The reference ensures pi_state lives, dropping the hb->lock - * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to - * close the races against futex_lock_pi(), but in case of - * _any_ fail we'll abort and retry the whole deal. + * By taking wait_lock while still holding hb->lock, we ensure + * there is no point where we hold neither; and therefore + * wake_futex_pi() must observe a state consistent with what we + * observed. */ - get_pi_state(pi_state); + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); spin_unlock(&hb->lock); ret = wake_futex_pi(uaddr, uval, pi_state); Patches currently in stable-queue which might be from ben@xxxxxxxxxxxxxxx are queue-4.9/futex-cleanup-refcounting.patch queue-4.9/futex-fix-more-put_pi_state-vs.-exit_pi_state_list-races.patch queue-4.9/futex-futex_unlock_pi-determinism.patch queue-4.9/futex-cleanup-variable-names-for-futex_top_waiter.patch queue-4.9/futex-don-t-enable-irqs-unconditionally-in-put_pi_state.patch queue-4.9/futex-fix-pi_state-owner-serialization.patch queue-4.9/futex-pull-rt_mutex_futex_unlock-out-from-under-hb-lock.patch