locking/rwsem: Make rwsem->owner an atomic_long_t

The rwsem->owner contains not just the task structure pointer, it also
holds some flags for storing the current state of the rwsem. Some of
the flags may have to be atomically updated. To reflect the new reality,
the owner is now changed to an atomic_long_t type.

New helper functions are added to properly separate out the task
structure pointer and the embedded flags.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
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: 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/20190520205918.22251-14-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/percpu-rwsem.h

@@ -117,7 +117,7 @@ static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
 	lock_release(&sem->rw_sem.dep_map, 1, ip);
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 	if (!read)
-		sem->rw_sem.owner = RWSEM_OWNER_UNKNOWN;
+		atomic_long_set(&sem->rw_sem.owner, RWSEM_OWNER_UNKNOWN);
 #endif
 }
 
@@ -127,7 +127,7 @@ static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
 	lock_acquire(&sem->rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 	if (!read)
-		sem->rw_sem.owner = current;
+		atomic_long_set(&sem->rw_sem.owner, (long)current);
 #endif
 }
 

include/linux/rwsem.h

@@ -35,10 +35,11 @@
 struct rw_semaphore {
 	atomic_long_t count;
 	/*
-	 * Write owner or one of the read owners. Can be used as a
-	 * speculative check to see if the owner is running on the cpu.
+	 * Write owner or one of the read owners as well flags regarding
+	 * the current state of the rwsem. Can be used as a speculative
+	 * check to see if the write owner is running on the cpu.
 	 */
-	struct task_struct *owner;
+	atomic_long_t owner;
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 	struct optimistic_spin_queue osq; /* spinner MCS lock */
 #endif
@@ -53,7 +54,7 @@ struct rw_semaphore {
  * Setting all bits of the owner field except bit 0 will indicate
  * that the rwsem is writer-owned with an unknown owner.
  */
-#define RWSEM_OWNER_UNKNOWN	((struct task_struct *)-2L)
+#define RWSEM_OWNER_UNKNOWN	(-2L)
 
 /* In all implementations count != 0 means locked */
 static inline int rwsem_is_locked(struct rw_semaphore *sem)
@@ -80,7 +81,7 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
 
 #define __RWSEM_INITIALIZER(name)				\
 	{ __RWSEM_INIT_COUNT(name),				\
-	  .owner = NULL,					\
+	  .owner = ATOMIC_LONG_INIT(0),				\
 	  .wait_list = LIST_HEAD_INIT((name).wait_list),	\
 	  .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock)	\
 	  __RWSEM_OPT_INIT(name)				\

kernel/locking/rwsem.c

@@ -64,7 +64,7 @@
 	if (!debug_locks_silent &&				\
 	    WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
 		#c, atomic_long_read(&(sem)->count),		\
-		(long)((sem)->owner), (long)current,		\
+		atomic_long_read(&(sem)->owner), (long)current,	\
 		list_empty(&(sem)->wait_list) ? "" : "not "))	\
 			debug_locks_off();			\
 	} while (0)
@@ -114,12 +114,20 @@
  */
 static inline void rwsem_set_owner(struct rw_semaphore *sem)
 {
-	WRITE_ONCE(sem->owner, current);
+	atomic_long_set(&sem->owner, (long)current);
 }
 
 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 {
-	WRITE_ONCE(sem->owner, NULL);
+	atomic_long_set(&sem->owner, 0);
+}
+
+/*
+ * Test the flags in the owner field.
+ */
+static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
+{
+	return atomic_long_read(&sem->owner) & flags;
 }
 
 /*
@@ -133,10 +141,9 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
 					    struct task_struct *owner)
 {
-	unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED
-						 | RWSEM_NONSPINNABLE;
+	unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED | RWSEM_NONSPINNABLE;
 
-	WRITE_ONCE(sem->owner, (struct task_struct *)val);
+	atomic_long_set(&sem->owner, val);
 }
 
 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
@@ -145,13 +152,20 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
 }
 
 /*
- * Return true if the a rwsem waiter can spin on the rwsem's owner
- * and steal the lock.
- * N.B. !owner is considered spinnable.
+ * Return true if the rwsem is owned by a reader.
  */
-static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
+static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
 {
-	return !((unsigned long)owner & RWSEM_NONSPINNABLE);
+#ifdef CONFIG_DEBUG_RWSEMS
+	/*
+	 * Check the count to see if it is write-locked.
+	 */
+	long count = atomic_long_read(&sem->count);
+
+	if (count & RWSEM_WRITER_MASK)
+		return false;
+#endif
+	return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
 }
 
 #ifdef CONFIG_DEBUG_RWSEMS
@@ -163,11 +177,13 @@ static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
  */
 static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
 {
-	unsigned long val = (unsigned long)current | RWSEM_READER_OWNED
-						   | RWSEM_NONSPINNABLE;
-	if (READ_ONCE(sem->owner) == (struct task_struct *)val)
-		cmpxchg_relaxed((unsigned long *)&sem->owner, val,
-				RWSEM_READER_OWNED | RWSEM_NONSPINNABLE);
+	unsigned long val = atomic_long_read(&sem->owner);
+
+	while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
+		if (atomic_long_try_cmpxchg(&sem->owner, &val,
+					    val & RWSEM_OWNER_FLAGS_MASK))
+			return;
+	}
 }
 #else
 static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
@@ -175,6 +191,28 @@ static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
 }
 #endif
 
+/*
+ * Return just the real task structure pointer of the owner
+ */
+static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
+{
+	return (struct task_struct *)
+		(atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
+}
+
+/*
+ * Return the real task structure pointer of the owner and the embedded
+ * flags in the owner. pflags must be non-NULL.
+ */
+static inline struct task_struct *
+rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
+{
+	unsigned long owner = atomic_long_read(&sem->owner);
+
+	*pflags = owner & RWSEM_OWNER_FLAGS_MASK;
+	return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
+}
+
 /*
  * Guide to the rw_semaphore's count field.
  *
@@ -208,7 +246,7 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
 	atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
 	raw_spin_lock_init(&sem->wait_lock);
 	INIT_LIST_HEAD(&sem->wait_list);
-	sem->owner = NULL;
+	atomic_long_set(&sem->owner, 0L);
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 	osq_lock_init(&sem->osq);
 #endif
@@ -511,9 +549,10 @@ static inline bool owner_on_cpu(struct task_struct *owner)
 static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 {
 	struct task_struct *owner;
+	unsigned long flags;
 	bool ret = true;
 
-	BUILD_BUG_ON(is_rwsem_owner_spinnable(RWSEM_OWNER_UNKNOWN));
+	BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & RWSEM_NONSPINNABLE));
 
 	if (need_resched()) {
 		lockevent_inc(rwsem_opt_fail);
@@ -522,11 +561,9 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 
 	preempt_disable();
 	rcu_read_lock();
-	owner = READ_ONCE(sem->owner);
-	if (owner) {
-		ret = is_rwsem_owner_spinnable(owner) &&
-		      owner_on_cpu(owner);
-	}
+	owner = rwsem_owner_flags(sem, &flags);
+	if ((flags & RWSEM_NONSPINNABLE) || (owner && !owner_on_cpu(owner)))
+		ret = false;
 	rcu_read_unlock();
 	preempt_enable();
 
@@ -553,25 +590,26 @@ enum owner_state {
 };
 #define OWNER_SPINNABLE		(OWNER_NULL | OWNER_WRITER)
 
-static inline enum owner_state rwsem_owner_state(unsigned long owner)
+static inline enum owner_state
+rwsem_owner_state(struct task_struct *owner, unsigned long flags)
 {
-	if (!owner)
-		return OWNER_NULL;
-
-	if (owner & RWSEM_NONSPINNABLE)
+	if (flags & RWSEM_NONSPINNABLE)
 		return OWNER_NONSPINNABLE;
 
-	if (owner & RWSEM_READER_OWNED)
+	if (flags & RWSEM_READER_OWNED)
 		return OWNER_READER;
 
-	return OWNER_WRITER;
+	return owner ? OWNER_WRITER : OWNER_NULL;
 }
 
 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);
+	struct task_struct *new, *owner;
+	unsigned long flags, new_flags;
+	enum owner_state state;
 
+	owner = rwsem_owner_flags(sem, &flags);
+	state = rwsem_owner_state(owner, flags);
 	if (state != OWNER_WRITER)
 		return state;
 
@@ -582,9 +620,9 @@ static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
 			break;
 		}
 
-		tmp = READ_ONCE(sem->owner);
-		if (tmp != owner) {
-			state = rwsem_owner_state((unsigned long)tmp);
+		new = rwsem_owner_flags(sem, &new_flags);
+		if ((new != owner) || (new_flags != flags)) {
+			state = rwsem_owner_state(new, new_flags);
 			break;
 		}
 
@@ -1001,8 +1039,7 @@ inline void __down_read(struct rw_semaphore *sem)
 	if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
 			&sem->count) & RWSEM_READ_FAILED_MASK)) {
 		rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
-		DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
-					RWSEM_READER_OWNED), sem);
+		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 	} else {
 		rwsem_set_reader_owned(sem);
 	}
@@ -1014,8 +1051,7 @@ static inline int __down_read_killable(struct rw_semaphore *sem)
 			&sem->count) & RWSEM_READ_FAILED_MASK)) {
 		if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
 			return -EINTR;
-		DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
-					RWSEM_READER_OWNED), sem);
+		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 	} else {
 		rwsem_set_reader_owned(sem);
 	}
@@ -1084,7 +1120,7 @@ inline void __up_read(struct rw_semaphore *sem)
 {
 	long tmp;
 
-	DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED), sem);
+	DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 	rwsem_clear_reader_owned(sem);
 	tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
 	if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
@@ -1103,8 +1139,8 @@ static inline void __up_write(struct rw_semaphore *sem)
 	 * sem->owner may differ from current if the ownership is transferred
 	 * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
 	 */
-	DEBUG_RWSEMS_WARN_ON((sem->owner != current) &&
-			    !((long)sem->owner & RWSEM_NONSPINNABLE), sem);
+	DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
+			    !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
 	rwsem_clear_owner(sem);
 	tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
 	if (unlikely(tmp & RWSEM_FLAG_WAITERS))
@@ -1125,7 +1161,7 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
 	 * read-locked region is ok to be re-ordered into the
 	 * write side. As such, rely on RELEASE semantics.
 	 */
-	DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
+	DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
 	tmp = atomic_long_fetch_add_release(
 		-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
 	rwsem_set_reader_owned(sem);
@@ -1296,8 +1332,7 @@ EXPORT_SYMBOL(down_write_killable_nested);
 
 void up_read_non_owner(struct rw_semaphore *sem)
 {
-	DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED),
-				sem);
+	DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 	__up_read(sem);
 }
 EXPORT_SYMBOL(up_read_non_owner);