locking/mutexes: Introduce cancelable MCS lock for adaptive spinning

Since we want a task waiting for a mutex_lock() to go to sleep and
reschedule on need_resched() we must be able to abort the
mcs_spin_lock() around the adaptive spin.

Therefore implement a cancelable mcs lock.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: chegu_vinod@hp.com
Cc: paulmck@linux.vnet.ibm.com
Cc: Waiman.Long@hp.com
Cc: torvalds@linux-foundation.org
Cc: tglx@linutronix.de
Cc: riel@redhat.com
Cc: akpm@linux-foundation.org
Cc: davidlohr@hp.com
Cc: hpa@zytor.com
Cc: andi@firstfloor.org
Cc: aswin@hp.com
Cc: scott.norton@hp.com
Cc: Jason Low <jason.low2@hp.com>
Link: http://lkml.kernel.org/n/tip-62hcl5wxydmjzd182zhvk89m@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

5 files changed, 202 insertions, 7 deletions

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index c482e1d2cc49..11692dea18aa 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -46,7 +46,7 @@
  * - detects multi-task circular deadlocks and prints out all affected
  *   locks and tasks (and only those tasks)
  */
-struct mcs_spinlock;
+struct optimistic_spin_queue;
 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 mcs_spinlock     *mcs_lock;      /* Spinner MCS lock */
+        struct optimistic_spin_queue    *osq;   /* Spinner MCS lock */
 #endif
 #ifdef CONFIG_DEBUG_MUTEXES
         const char              *name;
diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index baab8e5e7f66..2a9ee96ecf00 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -1,5 +1,5 @@
 
-obj-y += mutex.o semaphore.o rwsem.o lglock.o
+obj-y += mutex.o semaphore.o rwsem.o lglock.o mcs_spinlock.o
 
 ifdef CONFIG_FUNCTION_TRACER
 CFLAGS_REMOVE_lockdep.o = -pg
diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
new file mode 100644
index 000000000000..838dc9e00669
--- /dev/null
+++ b/kernel/locking/mcs_spinlock.c
@@ -0,0 +1,178 @@
+
+#include <linux/percpu.h>
+#include <linux/mutex.h>
+#include <linux/sched.h>
+#include "mcs_spinlock.h"
+
+#ifdef CONFIG_SMP
+
+/*
+ * An MCS like lock especially tailored for optimistic spinning for sleeping
+ * lock implementations (mutex, rwsem, etc).
+ *
+ * Using a single mcs node per CPU is safe because sleeping locks should not be
+ * called from interrupt context and we have preemption disabled while
+ * spinning.
+ */
+static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_queue, osq_node);
+
+/*
+ * 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_queue *
+osq_wait_next(struct optimistic_spin_queue **lock,
+              struct optimistic_spin_queue *node,
+              struct optimistic_spin_queue *prev)
+{
+        struct optimistic_spin_queue *next = NULL;
+
+        for (;;) {
+                if (*lock == node && cmpxchg(lock, node, prev) == node) {
+                        /*
+                         * We were the last queued, we moved @lock back. @prev
+                         * will now observe @lock and will complete its
+                         * unlock()/unqueue().
+                         */
+                        break;
+                }
+
+                /*
+                 * We must xchg() the @node->next value, because if we were to
+                 * leave it in, a concurrent unlock()/unqueue() from
+                 * @node->next might complete Step-A and think its @prev is
+                 * still valid.
+                 *
+                 * If the concurrent unlock()/unqueue() wins the race, we'll
+                 * wait for either @lock to point to us, through its Step-B, or
+                 * wait for a new @node->next from its Step-C.
+                 */
+                if (node->next) {
+                        next = xchg(&node->next, NULL);
+                        if (next)
+                                break;
+                }
+
+                arch_mutex_cpu_relax();
+        }
+
+        return next;
+}
+
+bool osq_lock(struct optimistic_spin_queue **lock)
+{
+        struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
+        struct optimistic_spin_queue *prev, *next;
+
+        node->locked = 0;
+        node->next = NULL;
+
+        node->prev = prev = xchg(lock, node);
+        if (likely(prev == NULL))
+                return true;
+
+        ACCESS_ONCE(prev->next) = node;
+
+        /*
+         * Normally @prev is untouchable after the above store; because at that
+         * moment unlock can proceed and wipe the node element from stack.
+         *
+         * However, since our nodes are static per-cpu storage, we're
+         * guaranteed their existence -- this allows us to apply
+         * cmpxchg in an attempt to undo our queueing.
+         */
+
+        while (!smp_load_acquire(&node->locked)) {
+                /*
+                 * If we need to reschedule bail... so we can block.
+                 */
+                if (need_resched())
+                        goto unqueue;
+
+                arch_mutex_cpu_relax();
+        }
+        return true;
+
+unqueue:
+        /*
+         * Step - A  -- stabilize @prev
+         *
+         * Undo our @prev->next assignment; this will make @prev's
+         * unlock()/unqueue() wait for a next pointer since @lock points to us
+         * (or later).
+         */
+
+        for (;;) {
+                if (prev->next == node &&
+                    cmpxchg(&prev->next, node, NULL) == node)
+                        break;
+
+                /*
+                 * We can only fail the cmpxchg() racing against an unlock(),
+                 * in which case we should observe @node->locked becomming
+                 * true.
+                 */
+                if (smp_load_acquire(&node->locked))
+                        return true;
+
+                arch_mutex_cpu_relax();
+
+                /*
+                 * Or we race against a concurrent unqueue()'s step-B, in which
+                 * case its step-C will write us a new @node->prev pointer.
+                 */
+                prev = ACCESS_ONCE(node->prev);
+        }
+
+        /*
+         * Step - B -- stabilize @next
+         *
+         * Similar to unlock(), wait for @node->next or move @lock from @node
+         * back to @prev.
+         */
+
+        next = osq_wait_next(lock, node, prev);
+        if (!next)
+                return false;
+
+        /*
+         * Step - C -- unlink
+         *
+         * @prev is stable because its still waiting for a new @prev->next
+         * pointer, @next is stable because our @node->next pointer is NULL and
+         * it will wait in Step-A.
+         */
+
+        ACCESS_ONCE(next->prev) = prev;
+        ACCESS_ONCE(prev->next) = next;
+
+        return false;
+}
+
+void osq_unlock(struct optimistic_spin_queue **lock)
+{
+        struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
+        struct optimistic_spin_queue *next;
+
+        /*
+         * Fast path for the uncontended case.
+         */
+        if (likely(cmpxchg(lock, node, NULL) == node))
+                return;
+
+        /*
+         * Second most likely case.
+         */
+        next = xchg(&node->next, NULL);
+        if (next) {
+                ACCESS_ONCE(next->locked) = 1;
+                return;
+        }
+
+        next = osq_wait_next(lock, node, NULL);
+        if (next)
+                ACCESS_ONCE(next->locked) = 1;
+}
+
+#endif
+
diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index f2a5c6360083..a2dbac4aca6b 100644
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -111,4 +111,19 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
         arch_mcs_spin_unlock_contended(&next->locked);
 }
 
+/*
+ * Cancellable version of the MCS lock above.
+ *
+ * Intended for adaptive spinning of sleeping locks:
+ * mutex_lock()/rwsem_down_{read,write}() etc.
+ */
+
+struct optimistic_spin_queue {
+        struct optimistic_spin_queue *next, *prev;
+        int locked; /* 1 if lock acquired */
+};
+
+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 dc3d6f2bbe2a..2670b84067d6 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -53,7 +53,7 @@ __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->mcs_lock = NULL;
+        lock->osq = NULL;
 #endif
 
         debug_mutex_init(lock, name, key);
@@ -403,7 +403,9 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
         if (!mutex_can_spin_on_owner(lock))
                 goto slowpath;
 
-        mcs_spin_lock(&lock->mcs_lock, &node);
+        if (!osq_lock(&lock->osq))
+                goto slowpath;
+
         for (;;) {
                 struct task_struct *owner;
 
@@ -442,7 +444,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                         }
 
                         mutex_set_owner(lock);
-                        mcs_spin_unlock(&lock->mcs_lock, &node);
+                        osq_unlock(&lock->osq);
                         preempt_enable();
                         return 0;
                 }
@@ -464,7 +466,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                  */
                 arch_mutex_cpu_relax();
         }
-        mcs_spin_unlock(&lock->mcs_lock, &node);
+        osq_unlock(&lock->osq);
 slowpath:
 #endif
         spin_lock_mutex(&lock->wait_lock, flags);