7 files changed, 68 insertions, 17 deletions

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 885f3f56a77f..42aa9b9ecd5f 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -17,6 +17,7 @@
 #include <linux/lockdep.h>
 #include <linux/atomic.h>
 #include <asm/processor.h>
+#include <linux/osq_lock.h>
 
 /*
  * Simple, straightforward mutexes with strict semantics:
@@ -46,7 +47,6 @@
  * - detects multi-task circular deadlocks and prints out all affected
  *   locks and tasks (and only those tasks)
  */
-struct optimistic_spin_node;
 struct mutex {
         /* 1: unlocked, 0: locked, negative: locked, possible waiters */
         atomic_t                count;
@@ -56,7 +56,7 @@ struct mutex {
         struct task_struct      *owner;
 #endif
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
-        struct optimistic_spin_node     *osq;   /* Spinner MCS lock */
+        struct optimistic_spin_queue osq; /* Spinner MCS lock */
 #endif
 #ifdef CONFIG_DEBUG_MUTEXES
         const char              *name;
diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
new file mode 100644
index 000000000000..b001682bf7cb
--- /dev/null
+++ b/include/linux/osq_lock.h
@@ -0,0 +1,19 @@
+#ifndef __LINUX_OSQ_LOCK_H
+#define __LINUX_OSQ_LOCK_H
+
+/*
+ * An MCS like lock especially tailored for optimistic spinning for sleeping
+ * lock implementations (mutex, rwsem, etc).
+ */
+
+#define OSQ_UNLOCKED_VAL (0)
+
+struct optimistic_spin_queue {
+        /*
+         * Stores an encoded value of the CPU # of the tail node in the queue.
+         * If the queue is empty, then it's set to OSQ_UNLOCKED_VAL.
+         */
+        atomic_t tail;
+};
+
+#endif
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index ba3f108ddea1..9fdcdd03507d 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -13,10 +13,9 @@
 #include <linux/kernel.h>
 #include <linux/list.h>
 #include <linux/spinlock.h>
-
 #include <linux/atomic.h>
+#include <linux/osq_lock.h>
 
-struct optimistic_spin_node;
 struct rw_semaphore;
 
 #ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
@@ -33,7 +32,7 @@ struct rw_semaphore {
          * if the owner is running on the cpu.
          */
         struct task_struct *owner;
-        struct optimistic_spin_node *osq; /* spinner MCS lock */
+        struct optimistic_spin_queue osq; /* spinner MCS lock */
 #endif
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
         struct lockdep_map      dep_map;
@@ -70,7 +69,7 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
           __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),     \
           LIST_HEAD_INIT((name).wait_list),             \
           NULL, /* owner */                             \
-          NULL /* mcs lock */                           \
+          { ATOMIC_INIT(OSQ_UNLOCKED_VAL) } /* osq */   \
           __RWSEM_DEP_MAP_INIT(name) }
 #else
 #define __RWSEM_INITIALIZER(name)                       \
diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
index e9866f70e828..32fc16c0a545 100644
--- a/kernel/locking/mcs_spinlock.c
+++ b/kernel/locking/mcs_spinlock.c
@@ -17,18 +17,44 @@
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
 
 /*
+ * We use the value 0 to represent "no CPU", thus the encoded value
+ * will be the CPU number incremented by 1.
+ */
+static inline int encode_cpu(int cpu_nr)
+{
+        return cpu_nr + 1;
+}
+
+static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
+{
+        int cpu_nr = encoded_cpu_val - 1;
+
+        return per_cpu_ptr(&osq_node, cpu_nr);
+}
+
+/*
  * Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
  * Can return NULL in case we were the last queued and we updated @lock instead.
  */
 static inline struct optimistic_spin_node *
-osq_wait_next(struct optimistic_spin_node **lock,
+osq_wait_next(struct optimistic_spin_queue *lock,
               struct optimistic_spin_node *node,
               struct optimistic_spin_node *prev)
 {
         struct optimistic_spin_node *next = NULL;
+        int curr = encode_cpu(smp_processor_id());
+        int old;
+
+        /*
+         * If there is a prev node in queue, then the 'old' value will be
+         * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
+         * we're currently last in queue, then the queue will then become empty.
+         */
+        old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
 
         for (;;) {
-                if (*lock == node && cmpxchg(lock, node, prev) == node) {
+                if (atomic_read(&lock->tail) == curr &&
+                    atomic_cmpxchg(&lock->tail, curr, old) == curr) {
                         /*
                          * We were the last queued, we moved @lock back. @prev
                          * will now observe @lock and will complete its
@@ -59,18 +85,23 @@ osq_wait_next(struct optimistic_spin_node **lock,
         return next;
 }
 
-bool osq_lock(struct optimistic_spin_node **lock)
+bool osq_lock(struct optimistic_spin_queue *lock)
 {
         struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
         struct optimistic_spin_node *prev, *next;
+        int curr = encode_cpu(smp_processor_id());
+        int old;
 
         node->locked = 0;
         node->next = NULL;
+        node->cpu = curr;
 
-        node->prev = prev = xchg(lock, node);
-        if (likely(prev == NULL))
+        old = atomic_xchg(&lock->tail, curr);
+        if (old == OSQ_UNLOCKED_VAL)
                 return true;
 
+        prev = decode_cpu(old);
+        node->prev = prev;
         ACCESS_ONCE(prev->next) = node;
 
         /*
@@ -149,15 +180,16 @@ unqueue:
         return false;
 }
 
-void osq_unlock(struct optimistic_spin_node **lock)
+void osq_unlock(struct optimistic_spin_queue *lock)
 {
         struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
         struct optimistic_spin_node *next;
+        int curr = encode_cpu(smp_processor_id());
 
         /*
          * Fast path for the uncontended case.
          */
-        if (likely(cmpxchg(lock, node, NULL) == node))
+        if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr))
                 return;
 
         /*
diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index c99dc0052f49..74356dc0ce29 100644
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -121,9 +121,10 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 struct optimistic_spin_node {
         struct optimistic_spin_node *next, *prev;
         int locked; /* 1 if lock acquired */
+        int cpu; /* encoded CPU # value */
 };
 
-extern bool osq_lock(struct optimistic_spin_node **lock);
-extern void osq_unlock(struct optimistic_spin_node **lock);
+extern bool osq_lock(struct optimistic_spin_queue *lock);
+extern void osq_unlock(struct optimistic_spin_queue *lock);
 
 #endif /* __LINUX_MCS_SPINLOCK_H */
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index bc73d33c6760..d9b313906caa 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -60,7 +60,7 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
         INIT_LIST_HEAD(&lock->wait_list);
         mutex_clear_owner(lock);
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
-        lock->osq = NULL;
+        atomic_set(&lock->osq.tail, OSQ_UNLOCKED_VAL);
 #endif
 
         debug_mutex_init(lock, name, key);
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index c40c7d28661d..b77a6230bbf6 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -84,7 +84,7 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
         INIT_LIST_HEAD(&sem->wait_list);
 #ifdef CONFIG_SMP
         sem->owner = NULL;
-        sem->osq = NULL;
+        atomic_set(&sem->osq.tail, OSQ_UNLOCKED_VAL);
 #endif
 }