locking/rwsem: Make rwsem_spin_on_owner() return owner state

This patch modifies rwsem_spin_on_owner() to return four possible
values to better reflect the state of lock holder which enables us to
make a better decision of what to do next.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: huang ying <huang.ying.caritas@gmail.com>
Link: https://lkml.kernel.org/r/20190520205918.22251-7-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Author: Waiman-Long

kernel/locking/rwsem.c

@@ -414,17 +414,54 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 }
 
 /*
- * Return true only if we can still spin on the owner field of the rwsem.
+ * The rwsem_spin_on_owner() function returns the folowing 4 values
+ * depending on the lock owner state.
+ *   OWNER_NULL  : owner is currently NULL
+ *   OWNER_WRITER: when owner changes and is a writer
+ *   OWNER_READER: when owner changes and the new owner may be a reader.
+ *   OWNER_NONSPINNABLE:
+ *		   when optimistic spinning has to stop because either the
+ *		   owner stops running, is unknown, or its timeslice has
+ *		   been used up.
  */
-static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
+enum owner_state {
+	OWNER_NULL		= 1 << 0,
+	OWNER_WRITER		= 1 << 1,
+	OWNER_READER		= 1 << 2,
+	OWNER_NONSPINNABLE	= 1 << 3,
+};
+#define OWNER_SPINNABLE		(OWNER_NULL | OWNER_WRITER)
+
+static inline enum owner_state rwsem_owner_state(unsigned long owner)
 {
-	struct task_struct *owner = READ_ONCE(sem->owner);
+	if (!owner)
+		return OWNER_NULL;
 
-	if (!is_rwsem_owner_spinnable(owner))
-		return false;
+	if (owner & RWSEM_ANONYMOUSLY_OWNED)
+		return OWNER_NONSPINNABLE;
+
+	if (owner & RWSEM_READER_OWNED)
+		return OWNER_READER;
+
+	return OWNER_WRITER;
+}
+
+static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
+{
+	struct task_struct *tmp, *owner = READ_ONCE(sem->owner);
+	enum owner_state state = rwsem_owner_state((unsigned long)owner);
+
+	if (state != OWNER_WRITER)
+		return state;
 
 	rcu_read_lock();
-	while (owner && (READ_ONCE(sem->owner) == owner)) {
+	for (;;) {
+		tmp = READ_ONCE(sem->owner);
+		if (tmp != owner) {
+			state = rwsem_owner_state((unsigned long)tmp);
+			break;
+		}
+
 		/*
 		 * Ensure we emit the owner->on_cpu, dereference _after_
 		 * checking sem->owner still matches owner, if that fails,
@@ -433,24 +470,16 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
 		 */
 		barrier();
 
-		/*
-		 * abort spinning when need_resched or owner is not running or
-		 * owner's cpu is preempted.
-		 */
 		if (need_resched() || !owner_on_cpu(owner)) {
-			rcu_read_unlock();
-			return false;
+			state = OWNER_NONSPINNABLE;
+			break;
 		}
 
 		cpu_relax();
 	}
 	rcu_read_unlock();
 
-	/*
-	 * If there is a new owner or the owner is not set, we continue
-	 * spinning.
-	 */
-	return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
+	return state;
 }
 
 static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
@@ -473,7 +502,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 	 *  2) readers own the lock as we can't determine if they are
 	 *     actively running or not.
 	 */
-	while (rwsem_spin_on_owner(sem)) {
+	while (rwsem_spin_on_owner(sem) & OWNER_SPINNABLE) {
 		/*
 		 * Try to acquire the lock
 		 */