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

Pull core locking changes from Ingo Molnar:
 "Main changes:

   - jump label asm preparatory work for PowerPC (Anton Blanchard)

   - rwsem optimizations and cleanups (Davidlohr Bueso)

   - mutex optimizations and cleanups (Jason Low)

   - futex fix (Oleg Nesterov)

   - remove broken atomicity checks from {READ,WRITE}_ONCE() (Peter
     Zijlstra)"

* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  powerpc, jump_label: Include linux/jump_label.h to get HAVE_JUMP_LABEL define
  jump_label: Allow jump labels to be used in assembly
  jump_label: Allow asm/jump_label.h to be included in assembly
  locking/mutex: Further simplify mutex_spin_on_owner()
  locking: Remove atomicy checks from {READ,WRITE}_ONCE
  locking/rtmutex: Rename argument in the rt_mutex_adjust_prio_chain() documentation as well
  locking/rwsem: Fix lock optimistic spinning when owner is not running
  locking: Remove ACCESS_ONCE() usage
  locking/rwsem: Check for active lock before bailing on spinning
  locking/rwsem: Avoid deceiving lock spinners
  locking/rwsem: Set lock ownership ASAP
  locking/rwsem: Document barrier need when waking tasks
  locking/futex: Check PF_KTHREAD rather than !p->mm to filter out kthreads
  locking/mutex: Refactor mutex_spin_on_owner()
  locking/mutex: In mutex_spin_on_owner(), return true when owner changes

Makefile

@@ -779,6 +779,7 @@ KBUILD_ARFLAGS := $(call ar-option,D)
 # check for 'asm goto'
 ifeq ($(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC)), y)
 	KBUILD_CFLAGS += -DCC_HAVE_ASM_GOTO
+	KBUILD_AFLAGS += -DCC_HAVE_ASM_GOTO
 endif
 
 include $(srctree)/scripts/Makefile.kasan

arch/arm/include/asm/jump_label.h

 #ifndef _ASM_ARM_JUMP_LABEL_H
 #define _ASM_ARM_JUMP_LABEL_H
 
-#ifdef __KERNEL__
+#ifndef __ASSEMBLY__
 
 #include <linux/types.h>
 
@@ -27,8 +27,6 @@ static __always_inline bool arch_static_branch(struct static_key *key)
 	return true;
 }
 
-#endif /* __KERNEL__ */
-
 typedef u32 jump_label_t;
 
 struct jump_entry {
@@ -37,4 +35,5 @@ struct jump_entry {
 	jump_label_t key;
 };
 
+#endif  /* __ASSEMBLY__ */
 #endif

arch/arm64/include/asm/jump_label.h

@@ -18,11 +18,12 @@
  */
 #ifndef __ASM_JUMP_LABEL_H
 #define __ASM_JUMP_LABEL_H
+
+#ifndef __ASSEMBLY__
+
 #include <linux/types.h>
 #include <asm/insn.h>
 
-#ifdef __KERNEL__
-
 #define JUMP_LABEL_NOP_SIZE		AARCH64_INSN_SIZE
 
 static __always_inline bool arch_static_branch(struct static_key *key)
@@ -39,8 +40,6 @@ static __always_inline bool arch_static_branch(struct static_key *key)
 	return true;
 }
 
-#endif /* __KERNEL__ */
-
 typedef u64 jump_label_t;
 
 struct jump_entry {
@@ -49,4 +48,5 @@ struct jump_entry {
 	jump_label_t key;
 };
 
+#endif  /* __ASSEMBLY__ */
 #endif	/* __ASM_JUMP_LABEL_H */

arch/mips/include/asm/jump_label.h

@@ -8,9 +8,9 @@
 #ifndef _ASM_MIPS_JUMP_LABEL_H
 #define _ASM_MIPS_JUMP_LABEL_H
 
-#include <linux/types.h>
+#ifndef __ASSEMBLY__
 
-#ifdef __KERNEL__
+#include <linux/types.h>
 
 #define JUMP_LABEL_NOP_SIZE 4
 
@@ -39,8 +39,6 @@ static __always_inline bool arch_static_branch(struct static_key *key)
 	return true;
 }
 
-#endif /* __KERNEL__ */
-
 #ifdef CONFIG_64BIT
 typedef u64 jump_label_t;
 #else
@@ -53,4 +51,5 @@ struct jump_entry {
 	jump_label_t key;
 };
 
+#endif  /* __ASSEMBLY__ */
 #endif /* _ASM_MIPS_JUMP_LABEL_H */

arch/powerpc/platforms/powernv/opal-wrappers.S

@@ -9,11 +9,11 @@
  * 2 of the License, or (at your option) any later version.
  */
 
+#include <linux/jump_label.h>
 #include <asm/ppc_asm.h>
 #include <asm/hvcall.h>
 #include <asm/asm-offsets.h>
 #include <asm/opal.h>
-#include <asm/jump_label.h>
 
 	.section	".text"
 

arch/powerpc/platforms/pseries/hvCall.S

@@ -7,12 +7,12 @@
  * as published by the Free Software Foundation; either version
  * 2 of the License, or (at your option) any later version.
  */
+#include <linux/jump_label.h>
 #include <asm/hvcall.h>
 #include <asm/processor.h>
 #include <asm/ppc_asm.h>
 #include <asm/asm-offsets.h>
 #include <asm/ptrace.h>
-#include <asm/jump_label.h>
 
 	.section	".text"
 	

arch/powerpc/platforms/pseries/lpar.c

@@ -26,7 +26,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/console.h>
 #include <linux/export.h>
-#include <linux/static_key.h>
+#include <linux/jump_label.h>
 #include <asm/processor.h>
 #include <asm/mmu.h>
 #include <asm/page.h>

arch/s390/include/asm/jump_label.h

 #ifndef _ASM_S390_JUMP_LABEL_H
 #define _ASM_S390_JUMP_LABEL_H
 
+#ifndef __ASSEMBLY__
+
 #include <linux/types.h>
 
 #define JUMP_LABEL_NOP_SIZE 6
@@ -39,4 +41,5 @@ struct jump_entry {
 	jump_label_t key;
 };
 
+#endif  /* __ASSEMBLY__ */
 #endif

arch/sparc/include/asm/jump_label.h

 #ifndef _ASM_SPARC_JUMP_LABEL_H
 #define _ASM_SPARC_JUMP_LABEL_H
 
-#ifdef __KERNEL__
+#ifndef __ASSEMBLY__
 
 #include <linux/types.h>
 
@@ -22,8 +22,6 @@ static __always_inline bool arch_static_branch(struct static_key *key)
 	return true;
 }
 
-#endif /* __KERNEL__ */
-
 typedef u32 jump_label_t;
 
 struct jump_entry {
@@ -32,4 +30,5 @@ struct jump_entry {
 	jump_label_t key;
 };
 
+#endif  /* __ASSEMBLY__ */
 #endif

arch/x86/include/asm/jump_label.h

 #ifndef _ASM_X86_JUMP_LABEL_H
 #define _ASM_X86_JUMP_LABEL_H
 
-#ifdef __KERNEL__
+#ifndef __ASSEMBLY__
 
 #include <linux/stringify.h>
 #include <linux/types.h>
@@ -30,8 +30,6 @@ static __always_inline bool arch_static_branch(struct static_key *key)
 	return true;
 }
 
-#endif /* __KERNEL__ */
-
 #ifdef CONFIG_X86_64
 typedef u64 jump_label_t;
 #else
@@ -44,4 +42,5 @@ struct jump_entry {
 	jump_label_t key;
 };
 
+#endif  /* __ASSEMBLY__ */
 #endif

include/linux/compiler.h

@@ -192,29 +192,16 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 
 #include <uapi/linux/types.h>
 
-static __always_inline void data_access_exceeds_word_size(void)
-#ifdef __compiletime_warning
-__compiletime_warning("data access exceeds word size and won't be atomic")
-#endif
-;
-
-static __always_inline void data_access_exceeds_word_size(void)
-{
-}
-
 static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
 {
 	switch (size) {
 	case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
 	case 2: *(__u16 *)res = *(volatile __u16 *)p; break;
 	case 4: *(__u32 *)res = *(volatile __u32 *)p; break;
-#ifdef CONFIG_64BIT
 	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;
-#endif
 	default:
 		barrier();
 		__builtin_memcpy((void *)res, (const void *)p, size);
-		data_access_exceeds_word_size();
 		barrier();
 	}
 }
@@ -225,13 +212,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
 	case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
 	case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
-#ifdef CONFIG_64BIT
 	case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
-#endif
 	default:
 		barrier();
 		__builtin_memcpy((void *)p, (const void *)res, size);
-		data_access_exceeds_word_size();
 		barrier();
 	}
 }

include/linux/jump_label.h

@@ -45,6 +45,12 @@
  * same as using STATIC_KEY_INIT_FALSE.
  */
 
+#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
+# define HAVE_JUMP_LABEL
+#endif
+
+#ifndef __ASSEMBLY__
+
 #include <linux/types.h>
 #include <linux/compiler.h>
 #include <linux/bug.h>
@@ -55,7 +61,7 @@ extern bool static_key_initialized;
 				    "%s used before call to jump_label_init", \
 				    __func__)
 
-#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
+#ifdef HAVE_JUMP_LABEL
 
 struct static_key {
 	atomic_t enabled;
@@ -66,13 +72,18 @@ struct static_key {
 #endif
 };
 
-# include <asm/jump_label.h>
-# define HAVE_JUMP_LABEL
 #else
 struct static_key {
 	atomic_t enabled;
 };
-#endif	/* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
+#endif	/* HAVE_JUMP_LABEL */
+#endif /* __ASSEMBLY__ */
+
+#ifdef HAVE_JUMP_LABEL
+#include <asm/jump_label.h>
+#endif
+
+#ifndef __ASSEMBLY__
 
 enum jump_label_type {
 	JUMP_LABEL_DISABLE = 0,
@@ -203,3 +214,5 @@ static inline bool static_key_enabled(struct static_key *key)
 }
 
 #endif	/* _LINUX_JUMP_LABEL_H */
+
+#endif /* __ASSEMBLY__ */

include/linux/seqlock.h

@@ -108,7 +108,7 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
 	unsigned ret;
 
 repeat:
-	ret = ACCESS_ONCE(s->sequence);
+	ret = READ_ONCE(s->sequence);
 	if (unlikely(ret & 1)) {
 		cpu_relax();
 		goto repeat;
@@ -127,7 +127,7 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
  */
 static inline unsigned raw_read_seqcount(const seqcount_t *s)
 {
-	unsigned ret = ACCESS_ONCE(s->sequence);
+	unsigned ret = READ_ONCE(s->sequence);
 	smp_rmb();
 	return ret;
 }
@@ -179,7 +179,7 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
  */
 static inline unsigned raw_seqcount_begin(const seqcount_t *s)
 {
-	unsigned ret = ACCESS_ONCE(s->sequence);
+	unsigned ret = READ_ONCE(s->sequence);
 	smp_rmb();
 	return ret & ~1;
 }

kernel/futex.c

@@ -900,7 +900,7 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
 	if (!p)
 		return -ESRCH;
 
-	if (!p->mm) {
+	if (unlikely(p->flags & PF_KTHREAD)) {
 		put_task_struct(p);
 		return -EPERM;
 	}

kernel/locking/mcs_spinlock.h

@@ -78,7 +78,7 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 		 */
 		return;
 	}
-	ACCESS_ONCE(prev->next) = node;
+	WRITE_ONCE(prev->next, node);
 
 	/* Wait until the lock holder passes the lock down. */
 	arch_mcs_spin_lock_contended(&node->locked);
@@ -91,7 +91,7 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 static inline
 void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 {
-	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
+	struct mcs_spinlock *next = READ_ONCE(node->next);
 
 	if (likely(!next)) {
 		/*
@@ -100,7 +100,7 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 		if (likely(cmpxchg(lock, node, NULL) == node))
 			return;
 		/* Wait until the next pointer is set */
-		while (!(next = ACCESS_ONCE(node->next)))
+		while (!(next = READ_ONCE(node->next)))
 			cpu_relax_lowlatency();
 	}
 

kernel/locking/mutex.c

@@ -25,7 +25,7 @@
 #include <linux/spinlock.h>
 #include <linux/interrupt.h>
 #include <linux/debug_locks.h>
-#include "mcs_spinlock.h"
+#include <linux/osq_lock.h>
 
 /*
  * In the DEBUG case we are using the "NULL fastpath" for mutexes,
@@ -217,44 +217,35 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
 }
 
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
-static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
-{
-	if (lock->owner != owner)
-		return false;
-
-	/*
-	 * Ensure we emit the owner->on_cpu, dereference _after_ checking
-	 * lock->owner still matches owner, if that fails, owner might
-	 * point to free()d memory, if it still matches, the rcu_read_lock()
-	 * ensures the memory stays valid.
-	 */
-	barrier();
-
-	return owner->on_cpu;
-}
-
 /*
  * Look out! "owner" is an entirely speculative pointer
  * access and not reliable.
  */
 static noinline
-int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
+bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 {
+	bool ret = true;
+
 	rcu_read_lock();
-	while (owner_running(lock, owner)) {
-		if (need_resched())
+	while (lock->owner == owner) {
+		/*
+		 * Ensure we emit the owner->on_cpu, dereference _after_
+		 * checking lock->owner still matches owner. If that fails,
+		 * owner might point to freed memory. If it still matches,
+		 * the rcu_read_lock() ensures the memory stays valid.
+		 */
+		barrier();
+
+		if (!owner->on_cpu || need_resched()) {
+			ret = false;
 			break;
+		}
 
 		cpu_relax_lowlatency();
 	}
 	rcu_read_unlock();
 
-	/*
-	 * We break out the loop above on need_resched() and when the
-	 * owner changed, which is a sign for heavy contention. Return
-	 * success only when lock->owner is NULL.
-	 */
-	return lock->owner == NULL;
+	return ret;
 }
 
 /*
@@ -269,7 +260,7 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
 		return 0;
 
 	rcu_read_lock();
-	owner = ACCESS_ONCE(lock->owner);
+	owner = READ_ONCE(lock->owner);
 	if (owner)
 		retval = owner->on_cpu;
 	rcu_read_unlock();
@@ -343,7 +334,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
 			 * As such, when deadlock detection needs to be
 			 * performed the optimistic spinning cannot be done.
 			 */
-			if (ACCESS_ONCE(ww->ctx))
+			if (READ_ONCE(ww->ctx))
 				break;
 		}
 
@@ -351,7 +342,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
 		 * If there's an owner, wait for it to either
 		 * release the lock or go to sleep.
 		 */
-		owner = ACCESS_ONCE(lock->owner);
+		owner = READ_ONCE(lock->owner);
 		if (owner && !mutex_spin_on_owner(lock, owner))
 			break;
 
@@ -490,7 +481,7 @@ static inline int __sched
 __ww_mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx)
 {
 	struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
-	struct ww_acquire_ctx *hold_ctx = ACCESS_ONCE(ww->ctx);
+	struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
 
 	if (!hold_ctx)
 		return 0;

kernel/locking/osq_lock.c

@@ -98,7 +98,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 
 	prev = decode_cpu(old);
 	node->prev = prev;
-	ACCESS_ONCE(prev->next) = node;
+	WRITE_ONCE(prev->next, node);
 
 	/*
 	 * Normally @prev is untouchable after the above store; because at that
@@ -109,7 +109,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 * cmpxchg in an attempt to undo our queueing.
 	 */
 
-	while (!ACCESS_ONCE(node->locked)) {
+	while (!READ_ONCE(node->locked)) {
 		/*
 		 * If we need to reschedule bail... so we can block.
 		 */
@@ -148,7 +148,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 		 * 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);
+		prev = READ_ONCE(node->prev);
 	}
 
 	/*
@@ -170,8 +170,8 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 * it will wait in Step-A.
 	 */
 
-	ACCESS_ONCE(next->prev) = prev;
-	ACCESS_ONCE(prev->next) = next;
+	WRITE_ONCE(next->prev, prev);
+	WRITE_ONCE(prev->next, next);
 
 	return false;
 }
@@ -193,11 +193,11 @@ void osq_unlock(struct optimistic_spin_queue *lock)
 	node = this_cpu_ptr(&osq_node);
 	next = xchg(&node->next, NULL);
 	if (next) {
-		ACCESS_ONCE(next->locked) = 1;
+		WRITE_ONCE(next->locked, 1);
 		return;
 	}
 
 	next = osq_wait_next(lock, node, NULL);
 	if (next)
-		ACCESS_ONCE(next->locked) = 1;
+		WRITE_ONCE(next->locked, 1);
 }

kernel/locking/rtmutex.c

@@ -349,7 +349,7 @@ static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
  *
  * @task:	the task owning the mutex (owner) for which a chain walk is
  *		probably needed
- * @deadlock_detect: do we have to carry out deadlock detection?
+ * @chwalk:	do we have to carry out deadlock detection?
  * @orig_lock:	the mutex (can be NULL if we are walking the chain to recheck
  *		things for a task that has just got its priority adjusted, and
  *		is waiting on a mutex)

kernel/locking/rwsem-spinlock.c

@@ -85,6 +85,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 
 		list_del(&waiter->list);
 		tsk = waiter->task;
+		/*
+		 * 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().
+		 */
 		smp_mb();
 		waiter->task = NULL;
 		wake_up_process(tsk);

kernel/locking/rwsem-xadd.c

@@ -14,8 +14,9 @@
 #include <linux/init.h>
 #include <linux/export.h>
 #include <linux/sched/rt.h>
+#include <linux/osq_lock.h>
 
-#include "mcs_spinlock.h"
+#include "rwsem.h"
 
 /*
  * Guide to the rw_semaphore's count field for common values.
@@ -186,6 +187,13 @@ __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;
+		/*
+		 * 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().
+		 */
 		smp_mb();
 		waiter->task = NULL;
 		wake_up_process(tsk);
@@ -258,6 +266,7 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
 		    RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
 		if (!list_is_singular(&sem->wait_list))
 			rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
+		rwsem_set_owner(sem);
 		return true;
 	}
 
@@ -270,15 +279,17 @@ 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 = ACCESS_ONCE(sem->count);
+	long old, count = READ_ONCE(sem->count);
 
 	while (true) {
 		if (!(count == 0 || count == RWSEM_WAITING_BIAS))
 			return false;
 
 		old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
-		if (old == count)
+		if (old == count) {
+			rwsem_set_owner(sem);
 			return true;
+		}
 
 		count = old;
 	}
@@ -287,60 +298,67 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
 static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 {
 	struct task_struct *owner;
-	bool on_cpu = false;
+	bool ret = true;
 
 	if (need_resched())
 		return false;
 
 	rcu_read_lock();
-	owner = ACCESS_ONCE(sem->owner);
-	if (owner)
-		on_cpu = owner->on_cpu;
-	rcu_read_unlock();
-
+	owner = READ_ONCE(sem->owner);
+	if (!owner) {
+		long count = READ_ONCE(sem->count);
 		/*
 		 * If sem->owner is not set, yet we have just recently entered the
-	 * slowpath, then there is a possibility reader(s) may have the lock.
-	 * To be safe, avoid spinning in these situations.
+		 * 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.
 		 */
-	return on_cpu;
+		if (count & RWSEM_ACTIVE_MASK)
+			ret = false;
+		goto done;
+	}
+
+	ret = owner->on_cpu;
+done:
+	rcu_read_unlock();
+	return ret;
 }
 
-static inline bool owner_running(struct rw_semaphore *sem,
-				 struct task_struct *owner)
+static noinline
+bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
 {
-	if (sem->owner != owner)
-		return false;
+	long count;
 
+	rcu_read_lock();
+	while (sem->owner == owner) {
 		/*
-	 * Ensure we emit the owner->on_cpu, dereference _after_ checking
-	 * sem->owner still matches owner, if that fails, owner might
-	 * point to free()d memory, if it still matches, the rcu_read_lock()
-	 * ensures the memory stays valid.
+		 * Ensure we emit the owner->on_cpu, dereference _after_
+		 * checking sem->owner still matches owner, if that fails,
+		 * owner might point to free()d memory, if it still matches,
+		 * the rcu_read_lock() ensures the memory stays valid.
 		 */
 		barrier();
 
-	return owner->on_cpu;
-}
-
-static noinline
-bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
-{
-	rcu_read_lock();
-	while (owner_running(sem, owner)) {
-		if (need_resched())
-			break;
+		/* abort spinning when need_resched or owner is not running */
+		if (!owner->on_cpu || need_resched()) {
+			rcu_read_unlock();
+			return false;
+		}
 
 		cpu_relax_lowlatency();
 	}
 	rcu_read_unlock();
 
+	if (READ_ONCE(sem->owner))
+		return true; /* new owner, continue spinning */
+
 	/*
-	 * We break out the loop above on need_resched() or when the
-	 * owner changed, which is a sign for heavy contention. Return
-	 * success only when sem->owner is NULL.
+	 * When the owner is not set, the lock could be free or
+	 * held by readers. Check the counter to verify the
+	 * state.
 	 */
-	return sem->owner == NULL;
+	count = READ_ONCE(sem->count);
+	return (count == 0 || count == RWSEM_WAITING_BIAS);
 }
 
 static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
@@ -358,7 +376,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 		goto done;
 
 	while (true) {
-		owner = ACCESS_ONCE(sem->owner);
+		owner = READ_ONCE(sem->owner);
 		if (owner && !rwsem_spin_on_owner(sem, owner))
 			break;
 
@@ -432,7 +450,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
 
 	/* we're now waiting on the lock, but no longer actively locking */
 	if (waiting) {
-		count = ACCESS_ONCE(sem->count);
+		count = READ_ONCE(sem->count);
 
 		/*
 		 * If there were already threads queued before us and there are

kernel/locking/rwsem.c

@@ -9,29 +9,9 @@
 #include <linux/sched.h>
 #include <linux/export.h>
 #include <linux/rwsem.h>
-
 #include <linux/atomic.h>
 
-#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
-static inline void rwsem_set_owner(struct rw_semaphore *sem)
-{
-	sem->owner = current;
-}
-
-static inline void rwsem_clear_owner(struct rw_semaphore *sem)
-{
-	sem->owner = NULL;
-}
-
-#else
-static inline void rwsem_set_owner(struct rw_semaphore *sem)
-{
-}
-
-static inline void rwsem_clear_owner(struct rw_semaphore *sem)
-{
-}
-#endif
+#include "rwsem.h"
 
 /*
  * lock for reading

kernel/locking/rwsem.h

+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+static inline void rwsem_set_owner(struct rw_semaphore *sem)
+{
+	sem->owner = current;
+}
+
+static inline void rwsem_clear_owner(struct rw_semaphore *sem)
+{
+	sem->owner = NULL;
+}
+
+#else
+static inline void rwsem_set_owner(struct rw_semaphore *sem)
+{
+}
+
+static inline void rwsem_clear_owner(struct rw_semaphore *sem)
+{
+}
+#endif

lib/lockref.c

@@ -18,7 +18,7 @@
 #define CMPXCHG_LOOP(CODE, SUCCESS) do {					\
 	struct lockref old;							\
 	BUILD_BUG_ON(sizeof(old) != 8);						\
-	old.lock_count = ACCESS_ONCE(lockref->lock_count);			\
+	old.lock_count = READ_ONCE(lockref->lock_count);			\
 	while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) {  	\
 		struct lockref new = old, prev = old;				\
 		CODE								\