1 files changed, 147 insertions, 4 deletions

diff --git a/kernel/mutex.c b/kernel/mutex.c
index 52f23011b6e0..ad53a664f113 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -37,6 +37,12 @@
 # include <asm/mutex.h>
 #endif
 
+/*
+ * A negative mutex count indicates that waiters are sleeping waiting for the
+ * mutex.
+ */
+#define MUTEX_SHOW_NO_WAITER(mutex)     (atomic_read(&(mutex)->count) >= 0)
+
 void
 __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
 {
@@ -44,6 +50,9 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
         spin_lock_init(&lock->wait_lock);
         INIT_LIST_HEAD(&lock->wait_list);
         mutex_clear_owner(lock);
+#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
+        lock->spin_mlock = NULL;
+#endif
 
         debug_mutex_init(lock, name, key);
 }
@@ -95,6 +104,124 @@ void __sched mutex_lock(struct mutex *lock)
 EXPORT_SYMBOL(mutex_lock);
 #endif
 
+#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
+/*
+ * In order to avoid a stampede of mutex spinners from acquiring the mutex
+ * more or less simultaneously, the spinners need to acquire a MCS lock
+ * first before spinning on the owner field.
+ *
+ * We don't inline mspin_lock() so that perf can correctly account for the
+ * time spent in this lock function.
+ */
+struct mspin_node {
+        struct mspin_node *next ;
+        int               locked;       /* 1 if lock acquired */
+};
+#define MLOCK(mutex)    ((struct mspin_node **)&((mutex)->spin_mlock))
+
+static noinline
+void mspin_lock(struct mspin_node **lock, struct mspin_node *node)
+{
+        struct mspin_node *prev;
+
+        /* Init node */
+        node->locked = 0;
+        node->next   = NULL;
+
+        prev = xchg(lock, node);
+        if (likely(prev == NULL)) {
+                /* Lock acquired */
+                node->locked = 1;
+                return;
+        }
+        ACCESS_ONCE(prev->next) = node;
+        smp_wmb();
+        /* Wait until the lock holder passes the lock down */
+        while (!ACCESS_ONCE(node->locked))
+                arch_mutex_cpu_relax();
+}
+
+static void mspin_unlock(struct mspin_node **lock, struct mspin_node *node)
+{
+        struct mspin_node *next = ACCESS_ONCE(node->next);
+
+        if (likely(!next)) {
+                /*
+                 * Release the lock by setting it to NULL
+                 */
+                if (cmpxchg(lock, node, NULL) == node)
+                        return;
+                /* Wait until the next pointer is set */
+                while (!(next = ACCESS_ONCE(node->next)))
+                        arch_mutex_cpu_relax();
+        }
+        ACCESS_ONCE(next->locked) = 1;
+        smp_wmb();
+}
+
+/*
+ * Mutex spinning code migrated from kernel/sched/core.c
+ */
+
+static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
+{
+        if (lock->owner != owner)
+                return false;
+
+        /*
+         * Ensure we emit the owner->on_cpu, dereference _after_ checking
+         * lock->owner still matches owner, if that fails, owner might
+         * point to free()d memory, if it still matches, the rcu_read_lock()
+         * ensures the memory stays valid.
+         */
+        barrier();
+
+        return owner->on_cpu;
+}
+
+/*
+ * Look out! "owner" is an entirely speculative pointer
+ * access and not reliable.
+ */
+static noinline
+int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
+{
+        rcu_read_lock();
+        while (owner_running(lock, owner)) {
+                if (need_resched())
+                        break;
+
+                arch_mutex_cpu_relax();
+        }
+        rcu_read_unlock();
+
+        /*
+         * We break out the loop above on need_resched() and when the
+         * owner changed, which is a sign for heavy contention. Return
+         * success only when lock->owner is NULL.
+         */
+        return lock->owner == NULL;
+}
+
+/*
+ * Initial check for entering the mutex spinning loop
+ */
+static inline int mutex_can_spin_on_owner(struct mutex *lock)
+{
+        int retval = 1;
+
+        rcu_read_lock();
+        if (lock->owner)
+                retval = lock->owner->on_cpu;
+        rcu_read_unlock();
+        /*
+         * if lock->owner is not set, the mutex owner may have just acquired
+         * it and not set the owner yet or the mutex has been released.
+         */
+        return retval;
+}
+#endif
+
 static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);
 
 /**
@@ -158,25 +285,39 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
          *
          * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
          * to serialize everything.
+         *
+         * The mutex spinners are queued up using MCS lock so that only one
+         * spinner can compete for the mutex. However, if mutex spinning isn't
+         * going to happen, there is no point in going through the lock/unlock
+         * overhead.
          */
+        if (!mutex_can_spin_on_owner(lock))
+                goto slowpath;
 
         for (;;) {
                 struct task_struct *owner;
+                struct mspin_node  node;
 
                 /*
                  * If there's an owner, wait for it to either
                  * release the lock or go to sleep.
                  */
+                mspin_lock(MLOCK(lock), &node);
                 owner = ACCESS_ONCE(lock->owner);
-                if (owner && !mutex_spin_on_owner(lock, owner))
+                if (owner && !mutex_spin_on_owner(lock, owner)) {
+                        mspin_unlock(MLOCK(lock), &node);
                         break;
+                }
 
-                if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
+                if ((atomic_read(&lock->count) == 1) &&
+                    (atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
                         lock_acquired(&lock->dep_map, ip);
                         mutex_set_owner(lock);
+                        mspin_unlock(MLOCK(lock), &node);
                         preempt_enable();
                         return 0;
                 }
+                mspin_unlock(MLOCK(lock), &node);
 
                 /*
                  * When there's no owner, we might have preempted between the
@@ -195,6 +336,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                  */
                 arch_mutex_cpu_relax();
         }
+slowpath:
 #endif
         spin_lock_mutex(&lock->wait_lock, flags);
 
@@ -205,7 +347,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
         list_add_tail(&waiter.list, &lock->wait_list);
         waiter.task = task;
 
-        if (atomic_xchg(&lock->count, -1) == 1)
+        if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1))
                 goto done;
 
         lock_contended(&lock->dep_map, ip);
@@ -220,7 +362,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                  * that when we release the lock, we properly wake up the
                  * other waiters:
                  */
-                if (atomic_xchg(&lock->count, -1) == 1)
+                if (MUTEX_SHOW_NO_WAITER(lock) &&
+                   (atomic_xchg(&lock->count, -1) == 1))
                         break;
 
                 /*