From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
4.9.115-rt94-rc1 stable review patch.
If you have any objection to the inclusion of this patch, let me know.
--- 8< --- 8< --- 8< ---
[ Upstream commit c74aef2d06a9f59cece89093eecc552933cba72a ]
There was a reported suspicion about a race between exit_pi_state_list()
and put_pi_state(). The same report mentioned the comment with
put_pi_state() said it should be called with hb->lock held, and it no
longer is in all places.
As it turns out, the pi_state->owner serialization is indeed broken. As per
the new rules:
734009e96d19 ("futex: Change locking rules")
pi_state->owner should be serialized by pi_state->pi_mutex.wait_lock.
For the sites setting pi_state->owner we already hold wait_lock (where
required) but exit_pi_state_list() and put_pi_state() were not and
raced on clearing it.
Fixes: 734009e96d19 ("futex: Change locking rules")
Reported-by: Gratian Crisan <gratian.crisan@xxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: dvhart@xxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20170922154806.jd3ffltfk24m4o4y@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Julia Cartwright <julia@xxxxxx>
---
kernel/futex.c | 34 ++++++++++++++++++++++------------
1 file changed, 22 insertions(+), 12 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index 8ab0ddd4cf8f..47e42faad6c5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -819,8 +819,6 @@ static void get_pi_state(struct futex_pi_state *pi_state)
/*
* Drops a reference to the pi_state object and frees or caches it
* when the last reference is gone.
- *
- * Must be called with the hb lock held.
*/
static void put_pi_state(struct futex_pi_state *pi_state)
{
@@ -835,16 +833,22 @@ static void put_pi_state(struct futex_pi_state *pi_state)
* and has cleaned up the pi_state already
*/
if (pi_state->owner) {
- raw_spin_lock_irq(&pi_state->owner->pi_lock);
- list_del_init(&pi_state->list);
- raw_spin_unlock_irq(&pi_state->owner->pi_lock);
+ struct task_struct *owner;
- rt_mutex_proxy_unlock(&pi_state->pi_mutex, pi_state->owner);
+ raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+ owner = pi_state->owner;
+ if (owner) {
+ raw_spin_lock(&owner->pi_lock);
+ list_del_init(&pi_state->list);
+ raw_spin_unlock(&owner->pi_lock);
+ }
+ rt_mutex_proxy_unlock(&pi_state->pi_mutex, owner);
+ raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
}
- if (current->pi_state_cache)
+ if (current->pi_state_cache) {
kfree(pi_state);
- else {
+ } else {
/*
* pi_state->list is already empty.
* clear pi_state->owner.
@@ -903,14 +907,15 @@ void exit_pi_state_list(struct task_struct *curr)
raw_spin_unlock_irq(&curr->pi_lock);
spin_lock(&hb->lock);
-
- raw_spin_lock_irq(&curr->pi_lock);
+ raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+ raw_spin_lock(&curr->pi_lock);
/*
* We dropped the pi-lock, so re-check whether this
* task still owns the PI-state:
*/
if (head->next != next) {
- raw_spin_unlock_irq(&curr->pi_lock);
+ raw_spin_unlock(&curr->pi_lock);
+ raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);
raw_spin_lock_irq(&curr->pi_lock);
continue;
@@ -920,9 +925,10 @@ void exit_pi_state_list(struct task_struct *curr)
WARN_ON(list_empty(&pi_state->list));
list_del_init(&pi_state->list);
pi_state->owner = NULL;
- raw_spin_unlock_irq(&curr->pi_lock);
+ raw_spin_unlock(&curr->pi_lock);
get_pi_state(pi_state);
+ raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);
rt_mutex_futex_unlock(&pi_state->pi_mutex);
@@ -1204,6 +1210,10 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
WARN_ON(!list_empty(&pi_state->list));
list_add(&pi_state->list, &p->pi_state_list);
+ /*
+ * Assignment without holding pi_state->pi_mutex.wait_lock is safe
+ * because there is no concurrency as the object is not published yet.
+ */
pi_state->owner = p;
raw_spin_unlock_irq(&p->pi_lock);
--
2.18.0
