Merge branch 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking updates from Ingo Molnar: "The locking tree was busier in this cycle than the usual pattern - a couple of major projects happened to coincide. The main changes are: - implement the atomic_fetch_{add,sub,and,or,xor}() API natively across all SMP architectures (Peter Zijlstra) - add atomic_fetch_{inc/dec}() as well, using the generic primitives (Davidlohr Bueso) - optimize various aspects of rwsems (Jason Low, Davidlohr Bueso, Waiman Long) - optimize smp_cond_load_acquire() on arm64 and implement LSE based atomic{,64}_fetch_{add,sub,and,andnot,or,xor}{,_relaxed,_acquire,_release}() on arm64 (Will Deacon) - introduce smp_acquire__after_ctrl_dep() and fix various barrier mis-uses and bugs (Peter Zijlstra) - after discovering ancient spin_unlock_wait() barrier bugs in its implementation and usage, strengthen its semantics and update/fix usage sites (Peter Zijlstra) - optimize mutex_trylock() fastpath (Peter Zijlstra) - ... misc fixes and cleanups" * 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (67 commits) locking/atomic: Introduce inc/dec variants for the atomic_fetch_$op() API locking/barriers, arch/arm64: Implement LDXR+WFE based smp_cond_load_acquire() locking/static_keys: Fix non static symbol Sparse warning locking/qspinlock: Use __this_cpu_dec() instead of full-blown this_cpu_dec() locking/atomic, arch/tile: Fix tilepro build locking/atomic, arch/m68k: Remove comment locking/atomic, arch/arc: Fix build locking/Documentation: Clarify limited control-dependency scope locking/atomic, arch/rwsem: Employ atomic_long_fetch_add() locking/atomic, arch/qrwlock: Employ atomic_fetch_add_acquire() locking/atomic, arch/mips: Convert to _relaxed atomics locking/atomic, arch/alpha: Convert to _relaxed atomics locking/atomic: Remove the deprecated atomic_{set,clear}_mask() functions locking/atomic: Remove linux/atomic.h:atomic_fetch_or() locking/atomic: Implement atomic{,64,_long}_fetch_{add,sub,and,andnot,or,xor}{,_relaxed,_acquire,_release}() locking/atomic: Fix atomic64_relaxed() bits locking/atomic, arch/xtensa: Implement atomic_fetch_{add,sub,and,or,xor}() locking/atomic, arch/x86: Implement atomic{,64}_fetch_{add,sub,and,or,xor}() locking/atomic, arch/tile: Implement atomic{,64}_fetch_{add,sub,and,or,xor}() locking/atomic, arch/sparc: Implement atomic{,64}_fetch_{add,sub,and,or,xor}() ...
2016-07-25 12:41:29 -07:00
parent a2303849a6 f06628638c
commit c86ad14d30
108 changed files with 3341 additions and 1031 deletions
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -46,6 +46,7 @@
 #include <linux/gfp.h>
 #include <linux/kmemcheck.h>
 #include <linux/random.h>
+#include <linux/jhash.h>

 #include <asm/sections.h>

@@ -309,10 +310,14 @@ static struct hlist_head chainhash_table[CHAINHASH_SIZE];
 * It's a 64-bit hash, because it's important for the keys to be
 * unique.
 */
-#define iterate_chain_key(key1, key2) \
-	(((key1) << MAX_LOCKDEP_KEYS_BITS) ^ \
-	((key1) >> (64-MAX_LOCKDEP_KEYS_BITS)) ^ \
-	(key2))
+static inline u64 iterate_chain_key(u64 key, u32 idx)
+{
+	u32 k0 = key, k1 = key >> 32;
+
+	__jhash_mix(idx, k0, k1); /* Macro that modifies arguments! */
+
+	return k0 | (u64)k1 << 32;
+}

 void lockdep_off(void)
 {
--- a/kernel/locking/mutex-debug.h
+++ b/kernel/locking/mutex-debug.h
@@ -29,12 +29,12 @@ extern void debug_mutex_init(struct mutex *lock, const char *name,

 static inline void mutex_set_owner(struct mutex *lock)
 {
-	lock->owner = current;
+	WRITE_ONCE(lock->owner, current);
 }

 static inline void mutex_clear_owner(struct mutex *lock)
 {
-	lock->owner = NULL;
+	WRITE_ONCE(lock->owner, NULL);
 }

 #define spin_lock_mutex(lock, flags)			\
--- a/kernel/locking/mutex.h
+++ b/kernel/locking/mutex.h
@@ -17,14 +17,20 @@
 		__list_del((waiter)->list.prev, (waiter)->list.next)

 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
+/*
+ * The mutex owner can get read and written to locklessly.
+ * We should use WRITE_ONCE when writing the owner value to
+ * avoid store tearing, otherwise, a thread could potentially
+ * read a partially written and incomplete owner value.
+ */
 static inline void mutex_set_owner(struct mutex *lock)
 {
-	lock->owner = current;
+	WRITE_ONCE(lock->owner, current);
 }

 static inline void mutex_clear_owner(struct mutex *lock)
 {
-	lock->owner = NULL;
+	WRITE_ONCE(lock->owner, NULL);
 }
 #else
 static inline void mutex_set_owner(struct mutex *lock)
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -93,7 +93,7 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
 	 * that accesses can't leak upwards out of our subsequent critical
 	 * section in the case that the lock is currently held for write.
 	 */
-	cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts) - _QR_BIAS;
+	cnts = atomic_fetch_add_acquire(_QR_BIAS, &lock->cnts);
 	rspin_until_writer_unlock(lock, cnts);

 	/*
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -90,7 +90,7 @@ static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]);
 * therefore increment the cpu number by one.
 */

-static inline u32 encode_tail(int cpu, int idx)
+static inline __pure u32 encode_tail(int cpu, int idx)
 {
 	u32 tail;

@@ -103,7 +103,7 @@ static inline u32 encode_tail(int cpu, int idx)
 	return tail;
 }

-static inline struct mcs_spinlock *decode_tail(u32 tail)
+static inline __pure struct mcs_spinlock *decode_tail(u32 tail)
 {
 	int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
 	int idx = (tail &  _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
@@ -267,6 +267,63 @@ static __always_inline u32  __pv_wait_head_or_lock(struct qspinlock *lock,
 #define queued_spin_lock_slowpath	native_queued_spin_lock_slowpath
 #endif

+/*
+ * Various notes on spin_is_locked() and spin_unlock_wait(), which are
+ * 'interesting' functions:
+ *
+ * PROBLEM: some architectures have an interesting issue with atomic ACQUIRE
+ * operations in that the ACQUIRE applies to the LOAD _not_ the STORE (ARM64,
+ * PPC). Also qspinlock has a similar issue per construction, the setting of
+ * the locked byte can be unordered acquiring the lock proper.
+ *
+ * This gets to be 'interesting' in the following cases, where the /should/s
+ * end up false because of this issue.
+ *
+ *
+ * CASE 1:
+ *
+ * So the spin_is_locked() correctness issue comes from something like:
+ *
+ *   CPU0				CPU1
+ *
+ *   global_lock();			local_lock(i)
+ *     spin_lock(&G)			  spin_lock(&L[i])
+ *     for (i)				  if (!spin_is_locked(&G)) {
+ *       spin_unlock_wait(&L[i]);	    smp_acquire__after_ctrl_dep();
+ *					    return;
+ *					  }
+ *					  // deal with fail
+ *
+ * Where it is important CPU1 sees G locked or CPU0 sees L[i] locked such
+ * that there is exclusion between the two critical sections.
+ *
+ * The load from spin_is_locked(&G) /should/ be constrained by the ACQUIRE from
+ * spin_lock(&L[i]), and similarly the load(s) from spin_unlock_wait(&L[i])
+ * /should/ be constrained by the ACQUIRE from spin_lock(&G).
+ *
+ * Similarly, later stuff is constrained by the ACQUIRE from CTRL+RMB.
+ *
+ *
+ * CASE 2:
+ *
+ * For spin_unlock_wait() there is a second correctness issue, namely:
+ *
+ *   CPU0				CPU1
+ *
+ *   flag = set;
+ *   smp_mb();				spin_lock(&l)
+ *   spin_unlock_wait(&l);		if (!flag)
+ *					  // add to lockless list
+ *					spin_unlock(&l);
+ *   // iterate lockless list
+ *
+ * Which wants to ensure that CPU1 will stop adding bits to the list and CPU0
+ * will observe the last entry on the list (if spin_unlock_wait() had ACQUIRE
+ * semantics etc..)
+ *
+ * Where flag /should/ be ordered against the locked store of l.
+ */
+
 /*
 * queued_spin_lock_slowpath() can (load-)ACQUIRE the lock before
 * issuing an _unordered_ store to set _Q_LOCKED_VAL.
@@ -322,7 +379,7 @@ void queued_spin_unlock_wait(struct qspinlock *lock)
 		cpu_relax();

 done:
-	smp_rmb(); /* CTRL + RMB -> ACQUIRE */
+	smp_acquire__after_ctrl_dep();
 }
 EXPORT_SYMBOL(queued_spin_unlock_wait);
 #endif
@@ -418,7 +475,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 * sequentiality; this is because not all clear_pending_set_locked()
 	 * implementations imply full barriers.
 	 */
-	smp_cond_acquire(!(atomic_read(&lock->val) & _Q_LOCKED_MASK));
+	smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK));

 	/*
 	 * take ownership and clear the pending bit.
@@ -455,6 +512,8 @@ queue:
 	 * pending stuff.
 	 *
 	 * p,*,* -> n,*,*
+	 *
+	 * RELEASE, such that the stores to @node must be complete.
 	 */
 	old = xchg_tail(lock, tail);
 	next = NULL;
@@ -465,6 +524,15 @@ queue:
 	 */
 	if (old & _Q_TAIL_MASK) {
 		prev = decode_tail(old);
+		/*
+		 * The above xchg_tail() is also a load of @lock which generates,
+		 * through decode_tail(), a pointer.
+		 *
+		 * The address dependency matches the RELEASE of xchg_tail()
+		 * such that the access to @prev must happen after.
+		 */
+		smp_read_barrier_depends();
+
 		WRITE_ONCE(prev->next, node);

 		pv_wait_node(node, prev);
@@ -494,7 +562,7 @@ queue:
 	 *
 	 * The PV pv_wait_head_or_lock function, if active, will acquire
 	 * the lock and return a non-zero value. So we have to skip the
-	 * smp_cond_acquire() call. As the next PV queue head hasn't been
+	 * smp_cond_load_acquire() call. As the next PV queue head hasn't been
 	 * designated yet, there is no way for the locked value to become
 	 * _Q_SLOW_VAL. So both the set_locked() and the
 	 * atomic_cmpxchg_relaxed() calls will be safe.
@@ -505,7 +573,7 @@ queue:
 	if ((val = pv_wait_head_or_lock(lock, node)))
 		goto locked;

-	smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
+	val = smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_PENDING_MASK));

 locked:
 	/*
@@ -525,9 +593,9 @@ locked:
 			break;
 		}
 		/*
-		 * The smp_cond_acquire() call above has provided the necessary
-		 * acquire semantics required for locking. At most two
-		 * iterations of this loop may be ran.
+		 * The smp_cond_load_acquire() call above has provided the
+		 * necessary acquire semantics required for locking. At most
+		 * two iterations of this loop may be ran.
 		 */
 		old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
 		if (old == val)
@@ -551,7 +619,7 @@ release:
 	/*
 	 * release the node
 	 */
-	this_cpu_dec(mcs_nodes[0].count);
+	__this_cpu_dec(mcs_nodes[0].count);
 }
 EXPORT_SYMBOL(queued_spin_lock_slowpath);

--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -112,12 +112,12 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock)
 #else /* _Q_PENDING_BITS == 8 */
 static __always_inline void set_pending(struct qspinlock *lock)
 {
-	atomic_set_mask(_Q_PENDING_VAL, &lock->val);
+	atomic_or(_Q_PENDING_VAL, &lock->val);
 }

 static __always_inline void clear_pending(struct qspinlock *lock)
 {
-	atomic_clear_mask(_Q_PENDING_VAL, &lock->val);
+	atomic_andnot(_Q_PENDING_VAL, &lock->val);
 }

 static __always_inline int trylock_clear_pending(struct qspinlock *lock)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1478,7 +1478,7 @@ EXPORT_SYMBOL_GPL(rt_mutex_timed_lock);
 */
 int __sched rt_mutex_trylock(struct rt_mutex *lock)
 {
-	if (WARN_ON(in_irq() || in_nmi() || in_serving_softirq()))
+	if (WARN_ON_ONCE(in_irq() || in_nmi() || in_serving_softirq()))
 		return 0;

 	return rt_mutex_fasttrylock(lock, rt_mutex_slowtrylock);
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -80,7 +80,7 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
 	debug_check_no_locks_freed((void *)sem, sizeof(*sem));
 	lockdep_init_map(&sem->dep_map, name, key, 0);
 #endif
-	sem->count = RWSEM_UNLOCKED_VALUE;
+	atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
 	raw_spin_lock_init(&sem->wait_lock);
 	INIT_LIST_HEAD(&sem->wait_list);
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
@@ -114,12 +114,16 @@ enum rwsem_wake_type {
 *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
 *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
 * - there must be someone on the queue
- * - the spinlock must be held by the caller
+ * - the wait_lock must be held by the caller
+ * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
+ *   to actually wakeup the blocked task(s) and drop the reference count,
+ *   preferably when the wait_lock is released
 * - woken process blocks are discarded from the list after having task zeroed
- * - writers are only woken if downgrading is false
+ * - writers are only marked woken if downgrading is false
 */
 static struct rw_semaphore *
-__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
+__rwsem_mark_wake(struct rw_semaphore *sem,
+		  enum rwsem_wake_type wake_type, struct wake_q_head *wake_q)
 {
 	struct rwsem_waiter *waiter;
 	struct task_struct *tsk;
@@ -128,13 +132,16 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)

 	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
 	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
-		if (wake_type == RWSEM_WAKE_ANY)
-			/* Wake writer at the front of the queue, but do not
-			 * grant it the lock yet as we want other writers
-			 * to be able to steal it.  Readers, on the other hand,
-			 * will block as they will notice the queued writer.
+		if (wake_type == RWSEM_WAKE_ANY) {
+			/*
+			 * Mark writer at the front of the queue for wakeup.
+			 * Until the task is actually later awoken later by
+			 * the caller, other writers are able to steal it.
+			 * Readers, on the other hand, will block as they
+			 * will notice the queued writer.
 			 */
-			wake_up_process(waiter->task);
+			wake_q_add(wake_q, waiter->task);
+		}
 		goto out;
 	}

@@ -146,15 +153,27 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 	if (wake_type != RWSEM_WAKE_READ_OWNED) {
 		adjustment = RWSEM_ACTIVE_READ_BIAS;
 try_reader_grant:
-		oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
+		oldcount = atomic_long_fetch_add(adjustment, &sem->count);
+
 		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
-			/* A writer stole the lock. Undo our reader grant. */
-			if (rwsem_atomic_update(-adjustment, sem) &
-						RWSEM_ACTIVE_MASK)
+			/*
+			 * If the count is still less than RWSEM_WAITING_BIAS
+			 * after removing the adjustment, it is assumed that
+			 * a writer has stolen the lock. We have to undo our
+			 * reader grant.
+			 */
+			if (atomic_long_add_return(-adjustment, &sem->count) <
+			    RWSEM_WAITING_BIAS)
 				goto out;
 			/* Last active locker left. Retry waking readers. */
 			goto try_reader_grant;
 		}
+		/*
+		 * It is not really necessary to set it to reader-owned here,
+		 * but it gives the spinners an early indication that the
+		 * readers now have the lock.
+		 */
+		rwsem_set_reader_owned(sem);
 	}

 	/* Grant an infinite number of read locks to the readers at the front
@@ -179,7 +198,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 		adjustment -= RWSEM_WAITING_BIAS;

 	if (adjustment)
-		rwsem_atomic_add(adjustment, sem);
+		atomic_long_add(adjustment, &sem->count);

 	next = sem->wait_list.next;
 	loop = woken;
@@ -187,17 +206,15 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 		waiter = list_entry(next, struct rwsem_waiter, list);
 		next = waiter->list.next;
 		tsk = waiter->task;
+
+		wake_q_add(wake_q, tsk);
 		/*
-		 * Make sure we do not wakeup the next reader before
-		 * setting the nil condition to grant the next reader;
-		 * otherwise we could miss the wakeup on the other
-		 * side and end up sleeping again. See the pairing
-		 * in rwsem_down_read_failed().
+		 * Ensure that the last operation is setting the reader
+		 * waiter to nil such that rwsem_down_read_failed() cannot
+		 * race with do_exit() by always holding a reference count
+		 * to the task to wakeup.
 		 */
-		smp_mb();
-		waiter->task = NULL;
-		wake_up_process(tsk);
-		put_task_struct(tsk);
+		smp_store_release(&waiter->task, NULL);
 	} while (--loop);

 	sem->wait_list.next = next;
@@ -216,11 +233,11 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 	long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
 	struct rwsem_waiter waiter;
 	struct task_struct *tsk = current;
+	WAKE_Q(wake_q);

 	/* set up my own style of waitqueue */
 	waiter.task = tsk;
 	waiter.type = RWSEM_WAITING_FOR_READ;
-	get_task_struct(tsk);

 	raw_spin_lock_irq(&sem->wait_lock);
 	if (list_empty(&sem->wait_list))
@@ -228,7 +245,7 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 	list_add_tail(&waiter.list, &sem->wait_list);

 	/* we're now waiting on the lock, but no longer actively locking */
-	count = rwsem_atomic_update(adjustment, sem);
+	count = atomic_long_add_return(adjustment, &sem->count);

 	/* If there are no active locks, wake the front queued process(es).
 	 *
@@ -238,9 +255,10 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 	if (count == RWSEM_WAITING_BIAS ||
 	    (count > RWSEM_WAITING_BIAS &&
 	     adjustment != -RWSEM_ACTIVE_READ_BIAS))
-		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
+		sem = __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

 	raw_spin_unlock_irq(&sem->wait_lock);
+	wake_up_q(&wake_q);

 	/* wait to be given the lock */
 	while (true) {
@@ -255,17 +273,29 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 }
 EXPORT_SYMBOL(rwsem_down_read_failed);

+/*
+ * This function must be called with the sem->wait_lock held to prevent
+ * race conditions between checking the rwsem wait list and setting the
+ * sem->count accordingly.
+ */
 static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
 {
 	/*
-	 * Try acquiring the write lock. Check count first in order
-	 * to reduce unnecessary expensive cmpxchg() operations.
+	 * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
 	 */
-	if (count == RWSEM_WAITING_BIAS &&
-	    cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS,
-		    RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
-		if (!list_is_singular(&sem->wait_list))
-			rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
+	if (count != RWSEM_WAITING_BIAS)
+		return false;
+
+	/*
+	 * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
+	 * are other tasks on the wait list, we need to add on WAITING_BIAS.
+	 */
+	count = list_is_singular(&sem->wait_list) ?
+			RWSEM_ACTIVE_WRITE_BIAS :
+			RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
+
+	if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
+							== RWSEM_WAITING_BIAS) {
 		rwsem_set_owner(sem);
 		return true;
 	}
@@ -279,13 +309,13 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
 */
 static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
 {
-	long old, count = READ_ONCE(sem->count);
+	long old, count = atomic_long_read(&sem->count);

 	while (true) {
 		if (!(count == 0 || count == RWSEM_WAITING_BIAS))
 			return false;

-		old = cmpxchg_acquire(&sem->count, count,
+		old = atomic_long_cmpxchg_acquire(&sem->count, count,
 				      count + RWSEM_ACTIVE_WRITE_BIAS);
 		if (old == count) {
 			rwsem_set_owner(sem);
@@ -306,16 +336,11 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)

 	rcu_read_lock();
 	owner = READ_ONCE(sem->owner);
-	if (!owner) {
-		long count = READ_ONCE(sem->count);
+	if (!rwsem_owner_is_writer(owner)) {
 		/*
-		 * If sem->owner is not set, yet we have just recently entered the
-		 * slowpath with the lock being active, then there is a possibility
-		 * reader(s) may have the lock. To be safe, bail spinning in these
-		 * situations.
+		 * Don't spin if the rwsem is readers owned.
 		 */
-		if (count & RWSEM_ACTIVE_MASK)
-			ret = false;
+		ret = !rwsem_owner_is_reader(owner);
 		goto done;
 	}

@@ -325,10 +350,15 @@ done:
 	return ret;
 }

-static noinline
-bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
+/*
+ * Return true only if we can still spin on the owner field of the rwsem.
+ */
+static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
 {
-	long count;
+	struct task_struct *owner = READ_ONCE(sem->owner);
+
+	if (!rwsem_owner_is_writer(owner))
+		goto out;

 	rcu_read_lock();
 	while (sem->owner == owner) {
@@ -349,22 +379,16 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
 		cpu_relax_lowlatency();
 	}
 	rcu_read_unlock();
-
-	if (READ_ONCE(sem->owner))
-		return true; /* new owner, continue spinning */
-
+out:
 	/*
-	 * When the owner is not set, the lock could be free or
-	 * held by readers. Check the counter to verify the
-	 * state.
+	 * If there is a new owner or the owner is not set, we continue
+	 * spinning.
 	 */
-	count = READ_ONCE(sem->count);
-	return (count == 0 || count == RWSEM_WAITING_BIAS);
+	return !rwsem_owner_is_reader(READ_ONCE(sem->owner));
 }

 static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 {
-	struct task_struct *owner;
 	bool taken = false;

 	preempt_disable();
@@ -376,12 +400,17 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 	if (!osq_lock(&sem->osq))
 		goto done;

-	while (true) {
-		owner = READ_ONCE(sem->owner);
-		if (owner && !rwsem_spin_on_owner(sem, owner))
-			break;
-
-		/* wait_lock will be acquired if write_lock is obtained */
+	/*
+	 * Optimistically spin on the owner field and attempt to acquire the
+	 * lock whenever the owner changes. Spinning will be stopped when:
+	 *  1) the owning writer isn't running; or
+	 *  2) readers own the lock as we can't determine if they are
+	 *     actively running or not.
+	 */
+	while (rwsem_spin_on_owner(sem)) {
+		/*
+		 * Try to acquire the lock
+		 */
 		if (rwsem_try_write_lock_unqueued(sem)) {
 			taken = true;
 			break;
@@ -393,7 +422,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 		 * we're an RT task that will live-lock because we won't let
 		 * the owner complete.
 		 */
-		if (!owner && (need_resched() || rt_task(current)))
+		if (!sem->owner && (need_resched() || rt_task(current)))
 			break;

 		/*
@@ -440,9 +469,10 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
 	bool waiting = true; /* any queued threads before us */
 	struct rwsem_waiter waiter;
 	struct rw_semaphore *ret = sem;
+	WAKE_Q(wake_q);

 	/* undo write bias from down_write operation, stop active locking */
-	count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
+	count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);

 	/* do optimistic spinning and steal lock if possible */
 	if (rwsem_optimistic_spin(sem))
@@ -465,18 +495,29 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)

 	/* we're now waiting on the lock, but no longer actively locking */
 	if (waiting) {
-		count = READ_ONCE(sem->count);
+		count = atomic_long_read(&sem->count);

 		/*
 		 * If there were already threads queued before us and there are
 		 * no active writers, the lock must be read owned; so we try to
 		 * wake any read locks that were queued ahead of us.
 		 */
-		if (count > RWSEM_WAITING_BIAS)
-			sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
+		if (count > RWSEM_WAITING_BIAS) {
+			WAKE_Q(wake_q);
+
+			sem = __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
+			/*
+			 * The wakeup is normally called _after_ the wait_lock
+			 * is released, but given that we are proactively waking
+			 * readers we can deal with the wake_q overhead as it is
+			 * similar to releasing and taking the wait_lock again
+			 * for attempting rwsem_try_write_lock().
+			 */
+			wake_up_q(&wake_q);
+		}

 	} else
-		count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
+		count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);

 	/* wait until we successfully acquire the lock */
 	set_current_state(state);
@@ -492,7 +533,7 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)

 			schedule();
 			set_current_state(state);
-		} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
+		} while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);

 		raw_spin_lock_irq(&sem->wait_lock);
 	}
@@ -507,10 +548,11 @@ out_nolock:
 	raw_spin_lock_irq(&sem->wait_lock);
 	list_del(&waiter.list);
 	if (list_empty(&sem->wait_list))
-		rwsem_atomic_update(-RWSEM_WAITING_BIAS, sem);
+		atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
 	else
-		__rwsem_do_wake(sem, RWSEM_WAKE_ANY);
+		__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
 	raw_spin_unlock_irq(&sem->wait_lock);
+	wake_up_q(&wake_q);

 	return ERR_PTR(-EINTR);
 }
@@ -537,6 +579,7 @@ __visible
 struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 {
 	unsigned long flags;
+	WAKE_Q(wake_q);

 	/*
 	 * If a spinner is present, it is not necessary to do the wakeup.
@@ -573,9 +616,10 @@ locked:

 	/* do nothing if list empty */
 	if (!list_empty(&sem->wait_list))
-		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
+		sem = __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

 	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+	wake_up_q(&wake_q);

 	return sem;
 }
@@ -590,14 +634,16 @@ __visible
 struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
 {
 	unsigned long flags;
+	WAKE_Q(wake_q);

 	raw_spin_lock_irqsave(&sem->wait_lock, flags);

 	/* do nothing if list empty */
 	if (!list_empty(&sem->wait_list))
-		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
+		sem = __rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);

 	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+	wake_up_q(&wake_q);

 	return sem;
 }
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -22,6 +22,7 @@ void __sched down_read(struct rw_semaphore *sem)
 	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);

 	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
+	rwsem_set_reader_owned(sem);
 }

 EXPORT_SYMBOL(down_read);
@@ -33,8 +34,10 @@ int down_read_trylock(struct rw_semaphore *sem)
 {
 	int ret = __down_read_trylock(sem);

-	if (ret == 1)
+	if (ret == 1) {
 		rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
+		rwsem_set_reader_owned(sem);
+	}
 	return ret;
 }

@@ -124,7 +127,7 @@ void downgrade_write(struct rw_semaphore *sem)
 	 * lockdep: a downgraded write will live on as a write
 	 * dependency.
 	 */
-	rwsem_clear_owner(sem);
+	rwsem_set_reader_owned(sem);
 	__downgrade_write(sem);
 }

@@ -138,6 +141,7 @@ void down_read_nested(struct rw_semaphore *sem, int subclass)
 	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);

 	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
+	rwsem_set_reader_owned(sem);
 }

 EXPORT_SYMBOL(down_read_nested);
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -1,14 +1,58 @@
+/*
+ * The owner field of the rw_semaphore structure will be set to
+ * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
+ * the owner field when it unlocks. A reader, on the other hand, will
+ * not touch the owner field when it unlocks.
+ *
+ * In essence, the owner field now has the following 3 states:
+ *  1) 0
+ *     - lock is free or the owner hasn't set the field yet
+ *  2) RWSEM_READER_OWNED
+ *     - lock is currently or previously owned by readers (lock is free
+ *       or not set by owner yet)
+ *  3) Other non-zero value
+ *     - a writer owns the lock
+ */
+#define RWSEM_READER_OWNED	((struct task_struct *)1UL)
+
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+/*
+ * All writes to owner are protected by WRITE_ONCE() to make sure that
+ * store tearing can't happen as optimistic spinners may read and use
+ * the owner value concurrently without lock. Read from owner, however,
+ * may not need READ_ONCE() as long as the pointer value is only used
+ * for comparison and isn't being dereferenced.
+ */
 static inline void rwsem_set_owner(struct rw_semaphore *sem)
 {
-	sem->owner = current;
+	WRITE_ONCE(sem->owner, current);
 }

 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 {
-	sem->owner = NULL;
+	WRITE_ONCE(sem->owner, NULL);
 }

+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+	/*
+	 * We check the owner value first to make sure that we will only
+	 * do a write to the rwsem cacheline when it is really necessary
+	 * to minimize cacheline contention.
+	 */
+	if (sem->owner != RWSEM_READER_OWNED)
+		WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
+}
+
+static inline bool rwsem_owner_is_writer(struct task_struct *owner)
+{
+	return owner && owner != RWSEM_READER_OWNED;
+}
+
+static inline bool rwsem_owner_is_reader(struct task_struct *owner)
+{
+	return owner == RWSEM_READER_OWNED;
+}
 #else
 static inline void rwsem_set_owner(struct rw_semaphore *sem)
 {
@@ -17,4 +61,8 @@ static inline void rwsem_set_owner(struct rw_semaphore *sem)
 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 {
 }
+
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+}
 #endif