[PATCH 4.4 05/11] futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()

Greg Kroah-Hartman
2021-08-08T07:23:30Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit cfafcd117da0216520568c195cb2f6cd1980c4bb ]

By changing futex_lock_pi() to use rt_mutex_*_proxy_lock() all wait_list
modifications are done under both hb->lock and wait_lock.

This closes the obvious interleave pattern between futex_lock_pi() and
futex_unlock_pi(), but not entirely so. See below:

Before:

futex_lock_pi() futex_unlock_pi()
unlock hb->lock

lock hb->lock
unlock hb->lock

lock rt_mutex->wait_lock
unlock rt_mutex_wait_lock
-EAGAIN

lock rt_mutex->wait_lock
list_add
unlock rt_mutex->wait_lock

schedule()

lock rt_mutex->wait_lock
list_del
unlock rt_mutex->wait_lock

<idem>
-EAGAIN

lock hb->lock

After:

futex_lock_pi() futex_unlock_pi()

lock hb->lock
lock rt_mutex->wait_lock
list_add
unlock rt_mutex->wait_lock
unlock hb->lock

schedule()
lock hb->lock
unlock hb->lock
lock hb->lock
lock rt_mutex->wait_lock
list_del
unlock rt_mutex->wait_lock

lock rt_mutex->wait_lock
unlock rt_mutex_wait_lock
-EAGAIN

unlock hb->lock

It does however solve the earlier starvation/live-lock scenario which got
introduced with the -EAGAIN since unlike the before scenario; where the
-EAGAIN happens while futex_unlock_pi() doesn't hold any locks; in the
after scenario it happens while futex_unlock_pi() actually holds a lock,
and then it is serialized on that lock.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: juri.lelli@arm.com
Cc: bigeasy@linutronix.de
Cc: xlpang@redhat.com
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: jdesfossez@efficios.com
Cc: dvhart@infradead.org
Cc: bristot@redhat.com
Link: http://lkml.kernel.org/r/20170322104152.062785528@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Joe Korty <joe.korty@concurrent-rt.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/futex.c | 77 ++++++++++++++++++++++++++++------------
kernel/locking/rtmutex.c | 26 +++----------
kernel/locking/rtmutex_common.h | 1
3 files changed, 62 insertions(+), 42 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2284,20 +2284,7 @@ queue_unlock(struct futex_hash_bucket *h
hb_waiters_dec(hb);
}

-/**
- * queue_me() - Enqueue the futex_q on the futex_hash_bucket
- * @q: The futex_q to enqueue
- * @hb: The destination hash bucket
- *
- * The hb->lock must be held by the caller, and is released here. A call to
- * queue_me() is typically paired with exactly one call to unqueue_me(). The
- * exceptions involve the PI related operations, which may use unqueue_me_pi()
- * or nothing if the unqueue is done as part of the wake process and the unqueue
- * state is implicit in the state of woken task (see futex_wait_requeue_pi() for
- * an example).
- */
-static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
- __releases(&hb->lock)
+static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
{
int prio;

@@ -2314,6 +2301,24 @@ static inline void queue_me(struct futex
plist_node_init(&q->list, prio);
plist_add(&q->list, &hb->chain);
q->task = current;
+}
+
+/**
+ * queue_me() - Enqueue the futex_q on the futex_hash_bucket
+ * @q: The futex_q to enqueue
+ * @hb: The destination hash bucket
+ *
+ * The hb->lock must be held by the caller, and is released here. A call to
+ * queue_me() is typically paired with exactly one call to unqueue_me(). The
+ * exceptions involve the PI related operations, which may use unqueue_me_pi()
+ * or nothing if the unqueue is done as part of the wake process and the unqueue
+ * state is implicit in the state of woken task (see futex_wait_requeue_pi() for
+ * an example).
+ */
+static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
+ __releases(&hb->lock)
+{
+ __queue_me(q, hb);
spin_unlock(&hb->lock);
}

@@ -2819,6 +2824,7 @@ static int futex_lock_pi(u32 __user *uad
{
struct hrtimer_sleeper timeout, *to = NULL;
struct task_struct *exiting = NULL;
+ struct rt_mutex_waiter rt_waiter;
struct futex_hash_bucket *hb;
struct futex_q q = futex_q_init;
int res, ret;
@@ -2879,25 +2885,52 @@ retry_private:
}
}

+ WARN_ON(!q.pi_state);
+
/*
* Only actually queue now that the atomic ops are done:
*/
- queue_me(&q, hb);
+ __queue_me(&q, hb);

- WARN_ON(!q.pi_state);
- /*
- * Block on the PI mutex:
- */
- if (!trylock) {
- ret = rt_mutex_timed_futex_lock(&q.pi_state->pi_mutex, to);
- } else {
+ if (trylock) {
ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
/* Fixup the trylock return value: */
ret = ret ? 0 : -EWOULDBLOCK;
+ goto no_block;
}

+ /*
+ * We must add ourselves to the rt_mutex waitlist while holding hb->lock
+ * such that the hb and rt_mutex wait lists match.
+ */
+ rt_mutex_init_waiter(&rt_waiter);
+ ret = rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
+ if (ret) {
+ if (ret == 1)
+ ret = 0;
+
+ goto no_block;
+ }
+
+ spin_unlock(q.lock_ptr);
+
+ if (unlikely(to))
+ hrtimer_start_expires(&to->timer, HRTIMER_MODE_ABS);
+
+ ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);
+
spin_lock(q.lock_ptr);
/*
+ * If we failed to acquire the lock (signal/timeout), we must
+ * first acquire the hb->lock before removing the lock from the
+ * rt_mutex waitqueue, such that we can keep the hb and rt_mutex
+ * wait lists consistent.
+ */
+ if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter))
+ ret = 0;
+
+no_block:
+ /*
* Fixup the pi_state owner and possibly acquire the lock if we
* haven't already.
*/
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1489,19 +1489,6 @@ int __sched rt_mutex_lock_interruptible(
EXPORT_SYMBOL_GPL(rt_mutex_lock_interruptible);

/*
- * Futex variant with full deadlock detection.
- * Futex variants must not use the fast-path, see __rt_mutex_futex_unlock().
- */
-int __sched rt_mutex_timed_futex_lock(struct rt_mutex *lock,
- struct hrtimer_sleeper *timeout)
-{
- might_sleep();
-
- 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)
@@ -1774,12 +1761,6 @@ int rt_mutex_wait_proxy_lock(struct rt_m
/* sleep on the mutex */
ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);

- /*
- * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
- * have to fix that up.
- */
- fixup_rt_mutex_waiters(lock);
-
raw_spin_unlock(&lock->wait_lock);

return ret;
@@ -1819,6 +1800,13 @@ bool rt_mutex_cleanup_proxy_lock(struct
fixup_rt_mutex_waiters(lock);
cleanup = true;
}
+
+ /*
+ * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
+ * have to fix that up.
+ */
+ fixup_rt_mutex_waiters(lock);
+
raw_spin_unlock_irq(&lock->wait_lock);

return cleanup;
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -111,7 +111,6 @@ extern int rt_mutex_wait_proxy_lock(stru
struct rt_mutex_waiter *waiter);
extern bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
struct rt_mutex_waiter *waiter);
-extern int rt_mutex_timed_futex_lock(struct rt_mutex *l, struct hrtimer_sleeper *to);
extern int rt_mutex_futex_trylock(struct rt_mutex *l);
extern int __rt_mutex_futex_trylock(struct rt_mutex *l);