# 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)
{
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);
}
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);
/**
*
* 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
*/
arch_mutex_cpu_relax();
}
+slowpath:
#endif
spin_lock_mutex(&lock->wait_lock, flags);
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);
* 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;
/*
projects / karo-tx-linux.git / blobdiff