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Subject[GIT pull] locking fixes for 4.14
Linus,

please pull the latest locking-urgent-for-linus git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking-urgent-for-linus

Two fixes for locking:

- Plug a hole the pi_stat->owner serialization which was changed recently
and failed to fixup two usage sites.

- Prevent reordering of the rwsem_has_spinner() check vs. the decrement
of rwsem count in up_write() which causes a missed wakeup.

Thanks,

tglx

------------------>
Peter Zijlstra (1):
futex: Fix pi_state->owner serialization

Prateek Sood (1):
locking/rwsem-xadd: Fix missed wakeup due to reordering of load


kernel/futex.c | 33 ++++++++++++++++++++++-----------
kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
2 files changed, 49 insertions(+), 11 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 3d38eaf05492..0518a0bfc746 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -821,8 +821,6 @@ 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)
{
@@ -837,16 +835,22 @@ 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.
@@ -907,13 +911,14 @@ 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(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);
continue;
}
@@ -922,9 +927,10 @@ 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);
@@ -1208,6 +1214,10 @@ 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);

@@ -2878,6 +2888,7 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);

+ /* drops pi_state->pi_mutex.wait_lock */
ret = wake_futex_pi(uaddr, uval, pi_state);

put_pi_state(pi_state);
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 02f660666ab8..1fefe6dcafd7 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
DEFINE_WAKE_Q(wake_q);

/*
+ * __rwsem_down_write_failed_common(sem)
+ * rwsem_optimistic_spin(sem)
+ * osq_unlock(sem->osq)
+ * ...
+ * atomic_long_add_return(&sem->count)
+ *
+ * - VS -
+ *
+ * __up_write()
+ * if (atomic_long_sub_return_release(&sem->count) < 0)
+ * rwsem_wake(sem)
+ * osq_is_locked(&sem->osq)
+ *
+ * And __up_write() must observe !osq_is_locked() when it observes the
+ * atomic_long_add_return() in order to not miss a wakeup.
+ *
+ * This boils down to:
+ *
+ * [S.rel] X = 1 [RmW] r0 = (Y += 0)
+ * MB RMB
+ * [RmW] Y += 1 [L] r1 = X
+ *
+ * exists (r0=1 /\ r1=0)
+ */
+ smp_rmb();
+
+ /*
* If a spinner is present, it is not necessary to do the wakeup.
* Try to do wakeup only if the trylock succeeds to minimize
* spinlock contention which may introduce too much delay in the
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