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Message-Id: <20210208145805.463603952@linuxfoundation.org>
Date: Mon, 8 Feb 2021 16:00:26 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
juri.lelli@....com, bigeasy@...utronix.de, xlpang@...hat.com,
rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
jdesfossez@...icios.com, dvhart@...radead.org, bristot@...hat.com,
Thomas Gleixner <tglx@...utronix.de>,
Lee Jones <lee.jones@...aro.org>
Subject: [PATCH 4.4 04/38] futex: Rework inconsistent rt_mutex/futex_q state
From: Lee Jones <lee.jones@...aro.org>
From: Peter Zijlstra <peterz@...radead.org>
[Upstream commit 73d786bd043ebc855f349c81ea805f6b11cbf2aa ]
There is a weird state in the futex_unlock_pi() path when it interleaves
with a concurrent futex_lock_pi() at the point where it drops hb->lock.
In this case, it can happen that the rt_mutex wait_list and the futex_q
disagree on pending waiters, in particular rt_mutex will find no pending
waiters where futex_q thinks there are. In this case the rt_mutex unlock
code cannot assign an owner.
The futex side fixup code has to cleanup the inconsistencies with quite a
bunch of interesting corner cases.
Simplify all this by changing wake_futex_pi() to return -EAGAIN when this
situation occurs. This then gives the futex_lock_pi() code the opportunity
to continue and the retried futex_unlock_pi() will now observe a coherent
state.
The only problem is that this breaks RT timeliness guarantees. That
is, consider the following scenario:
T1 and T2 are both pinned to CPU0. prio(T2) > prio(T1)
CPU0
T1
lock_pi()
queue_me() <- Waiter is visible
preemption
T2
unlock_pi()
loops with -EAGAIN forever
Which is undesirable for PI primitives. Future patches will rectify
this.
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: juri.lelli@....com
Cc: bigeasy@...utronix.de
Cc: xlpang@...hat.com
Cc: rostedt@...dmis.org
Cc: mathieu.desnoyers@...icios.com
Cc: jdesfossez@...icios.com
Cc: dvhart@...radead.org
Cc: bristot@...hat.com
Link: http://lkml.kernel.org/r/20170322104151.850383690@infradead.org
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
[Lee: Back-ported to solve a dependency]
Signed-off-by: Lee Jones <lee.jones@...aro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
kernel/futex.c | 52 +++++++++++++++-------------------------------------
1 file changed, 15 insertions(+), 37 deletions(-)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1389,12 +1389,19 @@ static int wake_futex_pi(u32 __user *uad
new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
/*
- * It is possible that the next waiter (the one that brought
- * this owner to the kernel) timed out and is no longer
- * waiting on the lock.
- */
- if (!new_owner)
- new_owner = this->task;
+ * When we interleave with futex_lock_pi() where it does
+ * rt_mutex_timed_futex_lock(), 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).
+ *
+ * When this happens, give up our locks and try again, giving the
+ * futex_lock_pi() instance time to complete, either by waiting on the
+ * rtmutex or removing itself from the futex queue.
+ */
+ if (!new_owner) {
+ raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+ return -EAGAIN;
+ }
/*
* We pass it to the next owner. The WAITERS bit is always
@@ -2337,7 +2344,6 @@ static long futex_wait_restart(struct re
*/
static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
{
- struct task_struct *owner;
int ret = 0;
if (locked) {
@@ -2351,43 +2357,15 @@ static int fixup_owner(u32 __user *uaddr
}
/*
- * Catch the rare case, where the lock was released when we were on the
- * way back before we locked the hash bucket.
- */
- if (q->pi_state->owner == current) {
- /*
- * Try to get the rt_mutex now. This might fail as some other
- * task acquired the rt_mutex after we removed ourself from the
- * rt_mutex waiters list.
- */
- if (rt_mutex_futex_trylock(&q->pi_state->pi_mutex)) {
- locked = 1;
- goto out;
- }
-
- /*
- * pi_state is incorrect, some other task did a lock steal and
- * we returned due to timeout or signal without taking the
- * rt_mutex. Too late.
- */
- raw_spin_lock(&q->pi_state->pi_mutex.wait_lock);
- owner = rt_mutex_owner(&q->pi_state->pi_mutex);
- if (!owner)
- owner = rt_mutex_next_owner(&q->pi_state->pi_mutex);
- raw_spin_unlock(&q->pi_state->pi_mutex.wait_lock);
- ret = fixup_pi_state_owner(uaddr, q, owner);
- goto out;
- }
-
- /*
* Paranoia check. If we did not take the lock, then we should not be
* the owner of the rt_mutex.
*/
- if (rt_mutex_owner(&q->pi_state->pi_mutex) == current)
+ if (rt_mutex_owner(&q->pi_state->pi_mutex) == current) {
printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
"pi-state %p\n", ret,
q->pi_state->pi_mutex.owner,
q->pi_state->owner);
+ }
out:
return ret ? ret : locked;
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