[PATCH -v6 04/13] futex,rt_mutex: Provide futex specific rt_mutex API

From: Peter Zijlstra
Date: Wed Mar 22 2017 - 06:46:05 EST


Part of what makes futex_unlock_pi() intricate is that
rt_mutex_futex_unlock() -> rt_mutex_slowunlock() can drop
rt_mutex::wait_lock.

This means we cannot rely on the atomicy of wait_lock, which we would
like to do in order to not rely on hb->lock so much.

The reason rt_mutex_slowunlock() needs to drop wait_lock is because it
can race with the rt_mutex fastpath, however futexes have their own
fast path.

Since futexes already have a bunch of separate rt_mutex accessors,
complete that set and implement a rt_mutex variant without fastpath
for them.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
kernel/futex.c | 30 ++++++++++-----------
kernel/locking/rtmutex.c | 55 +++++++++++++++++++++++++++++-----------
kernel/locking/rtmutex_common.h | 9 +++++-
3 files changed, 62 insertions(+), 32 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -916,7 +916,7 @@ void exit_pi_state_list(struct task_stru
pi_state->owner = NULL;
raw_spin_unlock_irq(&curr->pi_lock);

- rt_mutex_unlock(&pi_state->pi_mutex);
+ rt_mutex_futex_unlock(&pi_state->pi_mutex);

spin_unlock(&hb->lock);

@@ -1364,20 +1364,18 @@ static int wake_futex_pi(u32 __user *uad
pi_state->owner = new_owner;
raw_spin_unlock(&new_owner->pi_lock);

- raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
-
- deboost = rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
-
/*
- * First unlock HB so the waiter does not spin on it once he got woken
- * up. Second wake up the waiter before the priority is adjusted. If we
- * deboost first (and lose our higher priority), then the task might get
- * scheduled away before the wake up can take place.
+ * We've updated the uservalue, this unlock cannot fail.
*/
+ deboost = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
+
+ raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);
- wake_up_q(&wake_q);
- if (deboost)
+
+ if (deboost) {
+ wake_up_q(&wake_q);
rt_mutex_adjust_prio(current);
+ }

return 0;
}
@@ -2253,7 +2251,7 @@ static int fixup_owner(u32 __user *uaddr
* task acquired the rt_mutex after we removed ourself from the
* rt_mutex waiters list.
*/
- if (rt_mutex_trylock(&q->pi_state->pi_mutex)) {
+ if (rt_mutex_futex_trylock(&q->pi_state->pi_mutex)) {
locked = 1;
goto out;
}
@@ -2568,7 +2566,7 @@ static int futex_lock_pi(u32 __user *uad
if (!trylock) {
ret = rt_mutex_timed_futex_lock(&q.pi_state->pi_mutex, to);
} else {
- ret = rt_mutex_trylock(&q.pi_state->pi_mutex);
+ ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
/* Fixup the trylock return value: */
ret = ret ? 0 : -EWOULDBLOCK;
}
@@ -2591,7 +2589,7 @@ static int futex_lock_pi(u32 __user *uad
* it and return the fault to userspace.
*/
if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current))
- rt_mutex_unlock(&q.pi_state->pi_mutex);
+ rt_mutex_futex_unlock(&q.pi_state->pi_mutex);

/* Unqueue and drop the lock */
unqueue_me_pi(&q);
@@ -2898,7 +2896,7 @@ static int futex_wait_requeue_pi(u32 __u
spin_lock(q.lock_ptr);
ret = fixup_pi_state_owner(uaddr2, &q, current);
if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current)
- rt_mutex_unlock(&q.pi_state->pi_mutex);
+ rt_mutex_futex_unlock(&q.pi_state->pi_mutex);
/*
* Drop the reference to the pi state which
* the requeue_pi() code acquired for us.
@@ -2938,7 +2936,7 @@ static int futex_wait_requeue_pi(u32 __u
* userspace.
*/
if (ret && rt_mutex_owner(pi_mutex) == current)
- rt_mutex_unlock(pi_mutex);
+ rt_mutex_futex_unlock(pi_mutex);

/* Unqueue and drop the lock. */
unqueue_me_pi(&q);
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1488,15 +1488,23 @@ EXPORT_SYMBOL_GPL(rt_mutex_lock_interrup

/*
* Futex variant with full deadlock detection.
+ * Futex variants must not use the fast-path, see __rt_mutex_futex_unlock().
*/
-int rt_mutex_timed_futex_lock(struct rt_mutex *lock,
+int __sched rt_mutex_timed_futex_lock(struct rt_mutex *lock,
struct hrtimer_sleeper *timeout)
{
might_sleep();

- return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout,
- RT_MUTEX_FULL_CHAINWALK,
- rt_mutex_slowlock);
+ return rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE,
+ timeout, RT_MUTEX_FULL_CHAINWALK);
+}
+
+/*
+ * Futex variant, must not use fastpath.
+ */
+int __sched rt_mutex_futex_trylock(struct rt_mutex *lock)
+{
+ return rt_mutex_slowtrylock(lock);
}

/**
@@ -1555,19 +1563,38 @@ void __sched rt_mutex_unlock(struct rt_m
EXPORT_SYMBOL_GPL(rt_mutex_unlock);

/**
- * rt_mutex_futex_unlock - Futex variant of rt_mutex_unlock
- * @lock: the rt_mutex to be unlocked
- *
- * Returns: true/false indicating whether priority adjustment is
- * required or not.
+ * Futex variant, that since futex variants do not use the fast-path, can be
+ * simple and will not need to retry.
*/
-bool __sched rt_mutex_futex_unlock(struct rt_mutex *lock,
- struct wake_q_head *wqh)
+bool __sched __rt_mutex_futex_unlock(struct rt_mutex *lock,
+ struct wake_q_head *wake_q)
+{
+ lockdep_assert_held(&lock->wait_lock);
+
+ debug_rt_mutex_unlock(lock);
+
+ if (!rt_mutex_has_waiters(lock)) {
+ lock->owner = NULL;
+ return false; /* done */
+ }
+
+ mark_wakeup_next_waiter(wake_q, lock);
+ return true; /* deboost and wakeups */
+}
+
+void __sched rt_mutex_futex_unlock(struct rt_mutex *lock)
{
- if (likely(rt_mutex_cmpxchg_release(lock, current, NULL)))
- return false;
+ DEFINE_WAKE_Q(wake_q);
+ bool deboost;

- return rt_mutex_slowunlock(lock, wqh);
+ raw_spin_lock_irq(&lock->wait_lock);
+ deboost = __rt_mutex_futex_unlock(lock, &wake_q);
+ raw_spin_unlock_irq(&lock->wait_lock);
+
+ if (deboost) {
+ wake_up_q(&wake_q);
+ rt_mutex_adjust_prio(current);
+ }
}

/**
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -109,9 +109,14 @@ extern int rt_mutex_start_proxy_lock(str
extern int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
struct hrtimer_sleeper *to,
struct rt_mutex_waiter *waiter);
+
extern int rt_mutex_timed_futex_lock(struct rt_mutex *l, struct hrtimer_sleeper *to);
-extern bool rt_mutex_futex_unlock(struct rt_mutex *lock,
- struct wake_q_head *wqh);
+extern int rt_mutex_futex_trylock(struct rt_mutex *l);
+
+extern void rt_mutex_futex_unlock(struct rt_mutex *lock);
+extern bool __rt_mutex_futex_unlock(struct rt_mutex *lock,
+ struct wake_q_head *wqh);
+
extern void rt_mutex_adjust_prio(struct task_struct *task);

#ifdef CONFIG_DEBUG_RT_MUTEXES