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
