futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()

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>

diff --git a/kernel/futex.c b/kernel/futex.c
index 1cd8df7d3a43192c577478683b343cf6617587d5..eecce7bab86dd41412c1b1337391ab53d068c51c 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2099,20 +2099,7 @@ queue_unlock(struct futex_hash_bucket *hb)
        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;
 
@@ -2129,6 +2116,24 @@ static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
        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);
 }
 
@@ -2587,6 +2592,7 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
 {
        struct hrtimer_sleeper timeout, *to = NULL;
        struct futex_pi_state *pi_state = NULL;
+       struct rt_mutex_waiter rt_waiter;
        struct futex_hash_bucket *hb;
        struct futex_q q = futex_q_init;
        int res, ret;
@@ -2639,24 +2645,51 @@ 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.

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 1e8368db276eeee56f394716314c44d87827aae5..48418a1733b8538faa8145d000c532b032f23150 100644 (file)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1492,19 +1492,6 @@ int __sched rt_mutex_lock_interruptible(struct rt_mutex *lock)
 }
 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.
  */
@@ -1782,12 +1769,6 @@ int rt_mutex_wait_proxy_lock(struct rt_mutex *lock,
        /* 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_irq(&lock->wait_lock);
 
        return ret;
@@ -1827,6 +1808,13 @@ bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
                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;

diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index 35361e4dc773ce9217c247df6a11297f118cd929..1e93e15a0e452c27e4a84a01b67c86765001ecc4 100644 (file)
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -113,7 +113,6 @@ extern int rt_mutex_wait_proxy_lock(struct rt_mutex *lock,
 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 void rt_mutex_futex_unlock(struct rt_mutex *lock);