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

Pull locking updates from Ingo Molnar:

- improve rwsem scalability

- add uninitialized rwsem debugging check

- reduce lockdep's stacktrace memory usage and add diagnostics

- misc cleanups, code consolidation and constification

* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
mutex: Fix up mutex_waiter usage
locking/mutex: Use mutex flags macro instead of hard code
locking/mutex: Make __mutex_owner static to mutex.c
locking/qspinlock,x86: Clarify virt_spin_lock_key
locking/rwsem: Check for operations on an uninitialized rwsem
locking/rwsem: Make handoff writer optimistically spin on owner
locking/lockdep: Report more stack trace statistics
locking/lockdep: Reduce space occupied by stack traces
stacktrace: Constify 'entries' arguments
locking/lockdep: Make it clear that what lock_class::key points at is not modified

arch/x86/include/asm/qspinlock.h

···
 #endif
 
 #ifdef CONFIG_PARAVIRT
+/*
+ * virt_spin_lock_key - enables (by default) the virt_spin_lock() hijack.
+ *
+ * Native (and PV wanting native due to vCPU pinning) should disable this key.
+ * It is done in this backwards fashion to only have a single direction change,
+ * which removes ordering between native_pv_spin_init() and HV setup.
+ */
 DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
 
 void native_pv_lock_init(void) __init;
 
+/*
+ * Shortcut for the queued_spin_lock_slowpath() function that allows
+ * virt to hijack it.
+ *
+ * Returns:
+ *   true - lock has been negotiated, all done;
+ *   false - queued_spin_lock_slowpath() will do its thing.
+ */
 #define virt_spin_lock virt_spin_lock
 static inline bool virt_spin_lock(struct qspinlock *lock)
 {

include/linux/lockdep.h

···
 
 extern struct lock_class_key __lockdep_no_validate__;
 
-struct lock_trace {
-	unsigned int		nr_entries;
-	unsigned int		offset;
-};
+struct lock_trace;
 
 #define LOCKSTAT_POINTS		4
 
···
 	 */
 	struct list_head		locks_after, locks_before;
 
-	struct lockdep_subclass_key	*key;
+	const struct lockdep_subclass_key *key;
 	unsigned int			subclass;
 	unsigned int			dep_gen_id;
 
···
 	 * IRQ/softirq usage tracking bits:
 	 */
 	unsigned long			usage_mask;
-	struct lock_trace		usage_traces[XXX_LOCK_USAGE_STATES];
+	const struct lock_trace		*usage_traces[XXX_LOCK_USAGE_STATES];
 
 	/*
 	 * Generation counter, when doing certain classes of graph walking,
···
 	struct list_head		entry;
 	struct lock_class		*class;
 	struct lock_class		*links_to;
-	struct lock_trace		trace;
+	const struct lock_trace		*trace;
 	int				distance;
 
 	/*

include/linux/mutex.h

···
 };
 
 /*
- * Internal helper function; C doesn't allow us to hide it :/
- *
- * DO NOT USE (outside of mutex code).
- */
-static inline struct task_struct *__mutex_owner(struct mutex *lock)
-{
-	return (struct task_struct *)(atomic_long_read(&lock->owner) & ~0x07);
-}
-
-/*
  * This is the control structure for tasks blocked on mutex,
  * which resides on the blocked task's kernel stack:
  */
···
  *
  * Returns true if the mutex is locked, false if unlocked.
  */
-static inline bool mutex_is_locked(struct mutex *lock)
-{
-	return __mutex_owner(lock) != NULL;
-}
+extern bool mutex_is_locked(struct mutex *lock);
 
 /*
  * See kernel/locking/mutex.c for detailed documentation of these APIs.
···
  *  - MUTEX_TRYLOCK_SUCCESS   - lock acquired,
  *  - MUTEX_TRYLOCK_RECURSIVE - we already owned the lock.
  */
-static inline /* __deprecated */ __must_check enum mutex_trylock_recursive_enum
-mutex_trylock_recursive(struct mutex *lock)
-{
-	if (unlikely(__mutex_owner(lock) == current))
-		return MUTEX_TRYLOCK_RECURSIVE;
-
-	return mutex_trylock(lock);
-}
+extern /* __deprecated */ __must_check enum mutex_trylock_recursive_enum
+mutex_trylock_recursive(struct mutex *lock);
 
 #endif /* __LINUX_MUTEX_H */

include/linux/rwsem.h

···
 #endif
 	raw_spinlock_t wait_lock;
 	struct list_head wait_list;
+#ifdef CONFIG_DEBUG_RWSEMS
+	void *magic;
+#endif
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	dep_map;
 #endif
···
 # define __RWSEM_DEP_MAP_INIT(lockname)
 #endif
 
+#ifdef CONFIG_DEBUG_RWSEMS
+# define __DEBUG_RWSEM_INITIALIZER(lockname) , .magic = &lockname
+#else
+# define __DEBUG_RWSEM_INITIALIZER(lockname)
+#endif
+
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 #define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED
 #else
···
 	  .wait_list = LIST_HEAD_INIT((name).wait_list),	\
 	  .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock)	\
 	  __RWSEM_OPT_INIT(name)				\
+	  __DEBUG_RWSEM_INITIALIZER(name)			\
 	  __RWSEM_DEP_MAP_INIT(name) }
 
 #define DECLARE_RWSEM(name) \

include/linux/stacktrace.h

···
 struct pt_regs;
 
 #ifdef CONFIG_STACKTRACE
-void stack_trace_print(unsigned long *trace, unsigned int nr_entries,
+void stack_trace_print(const unsigned long *trace, unsigned int nr_entries,
 		       int spaces);
-int stack_trace_snprint(char *buf, size_t size, unsigned long *entries,
+int stack_trace_snprint(char *buf, size_t size, const unsigned long *entries,
 			unsigned int nr_entries, int spaces);
 unsigned int stack_trace_save(unsigned long *store, unsigned int size,
 			      unsigned int skipnr);

kernel/locking/lockdep.c

···
 unsigned long nr_stack_trace_entries;
 
 #ifdef CONFIG_PROVE_LOCKING
+/**
+ * struct lock_trace - single stack backtrace
+ * @hash_entry:	Entry in a stack_trace_hash[] list.
+ * @hash:	jhash() of @entries.
+ * @nr_entries:	Number of entries in @entries.
+ * @entries:	Actual stack backtrace.
+ */
+struct lock_trace {
+	struct hlist_node	hash_entry;
+	u32			hash;
+	u32			nr_entries;
+	unsigned long		entries[0] __aligned(sizeof(unsigned long));
+};
+#define LOCK_TRACE_SIZE_IN_LONGS				\
+	(sizeof(struct lock_trace) / sizeof(unsigned long))
 /*
- * Stack-trace: tightly packed array of stack backtrace
- * addresses. Protected by the graph_lock.
+ * Stack-trace: sequence of lock_trace structures. Protected by the graph_lock.
  */
 static unsigned long stack_trace[MAX_STACK_TRACE_ENTRIES];
+static struct hlist_head stack_trace_hash[STACK_TRACE_HASH_SIZE];
 
-static int save_trace(struct lock_trace *trace)
+static bool traces_identical(struct lock_trace *t1, struct lock_trace *t2)
 {
-	unsigned long *entries = stack_trace + nr_stack_trace_entries;
+	return t1->hash == t2->hash && t1->nr_entries == t2->nr_entries &&
+		memcmp(t1->entries, t2->entries,
+		       t1->nr_entries * sizeof(t1->entries[0])) == 0;
+}
+
+static struct lock_trace *save_trace(void)
+{
+	struct lock_trace *trace, *t2;
+	struct hlist_head *hash_head;
+	u32 hash;
 	unsigned int max_entries;
 
-	trace->offset = nr_stack_trace_entries;
-	max_entries = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries;
-	trace->nr_entries = stack_trace_save(entries, max_entries, 3);
-	nr_stack_trace_entries += trace->nr_entries;
+	BUILD_BUG_ON_NOT_POWER_OF_2(STACK_TRACE_HASH_SIZE);
+	BUILD_BUG_ON(LOCK_TRACE_SIZE_IN_LONGS >= MAX_STACK_TRACE_ENTRIES);
 
-	if (nr_stack_trace_entries >= MAX_STACK_TRACE_ENTRIES-1) {
+	trace = (struct lock_trace *)(stack_trace + nr_stack_trace_entries);
+	max_entries = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries -
+		LOCK_TRACE_SIZE_IN_LONGS;
+	trace->nr_entries = stack_trace_save(trace->entries, max_entries, 3);
+
+	if (nr_stack_trace_entries >= MAX_STACK_TRACE_ENTRIES -
+	    LOCK_TRACE_SIZE_IN_LONGS - 1) {
 		if (!debug_locks_off_graph_unlock())
-			return 0;
+			return NULL;
 
 		print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too low!");
 		dump_stack();
 
-		return 0;
+		return NULL;
 	}
 
-	return 1;
+	hash = jhash(trace->entries, trace->nr_entries *
+		     sizeof(trace->entries[0]), 0);
+	trace->hash = hash;
+	hash_head = stack_trace_hash + (hash & (STACK_TRACE_HASH_SIZE - 1));
+	hlist_for_each_entry(t2, hash_head, hash_entry) {
+		if (traces_identical(trace, t2))
+			return t2;
+	}
+	nr_stack_trace_entries += LOCK_TRACE_SIZE_IN_LONGS + trace->nr_entries;
+	hlist_add_head(&trace->hash_entry, hash_head);
+
+	return trace;
+}
+
+/* Return the number of stack traces in the stack_trace[] array. */
+u64 lockdep_stack_trace_count(void)
+{
+	struct lock_trace *trace;
+	u64 c = 0;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(stack_trace_hash); i++) {
+		hlist_for_each_entry(trace, &stack_trace_hash[i], hash_entry) {
+			c++;
+		}
+	}
+
+	return c;
+}
+
+/* Return the number of stack hash chains that have at least one stack trace. */
+u64 lockdep_stack_hash_count(void)
+{
+	u64 c = 0;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(stack_trace_hash); i++)
+		if (!hlist_empty(&stack_trace_hash[i]))
+			c++;
+
+	return c;
 }
 #endif
 
···
 };
 #endif
 
-const char * __get_key_name(struct lockdep_subclass_key *key, char *str)
+const char *__get_key_name(const struct lockdep_subclass_key *key, char *str)
 {
 	return kallsyms_lookup((unsigned long)key, NULL, NULL, NULL, str);
 }
···
 static int add_lock_to_list(struct lock_class *this,
 			    struct lock_class *links_to, struct list_head *head,
 			    unsigned long ip, int distance,
-			    struct lock_trace *trace)
+			    const struct lock_trace *trace)
 {
 	struct lock_list *entry;
 	/*
···
 	entry->class = this;
 	entry->links_to = links_to;
 	entry->distance = distance;
-	entry->trace = *trace;
+	entry->trace = trace;
 	/*
 	 * Both allocation and removal are done under the graph lock; but
 	 * iteration is under RCU-sched; see look_up_lock_class() and
···
 
 }
 
-static void print_lock_trace(struct lock_trace *trace, unsigned int spaces)
+static void print_lock_trace(const struct lock_trace *trace,
+			     unsigned int spaces)
 {
-	unsigned long *entries = stack_trace + trace->offset;
-
-	stack_trace_print(entries, trace->nr_entries, spaces);
+	stack_trace_print(trace->entries, trace->nr_entries, spaces);
 }
 
 /*
···
 	printk("\n-> #%u", depth);
 	print_lock_name(target->class);
 	printk(KERN_CONT ":\n");
-	print_lock_trace(&target->trace, 6);
+	print_lock_trace(target->trace, 6);
 }
 
 static void
···
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
 		return;
 
-	if (!save_trace(&this->trace))
+	this->trace = save_trace();
+	if (!this->trace)
 		return;
 
 	depth = get_lock_depth(target);
···
  */
 static noinline int
 check_noncircular(struct held_lock *src, struct held_lock *target,
-		  struct lock_trace *trace)
+		  struct lock_trace **const trace)
 {
 	int ret;
 	struct lock_list *uninitialized_var(target_entry);
···
 	ret = check_path(hlock_class(target), &src_entry, &target_entry);
 
 	if (unlikely(!ret)) {
-		if (!trace->nr_entries) {
+		if (!*trace) {
 			/*
 			 * If save_trace fails here, the printing might
 			 * trigger a WARN but because of the !nr_entries it
 			 * should not do bad things.
 			 */
-			save_trace(trace);
+			*trace = save_trace();
 		}
 
 		print_circular_bug(&src_entry, target_entry, src, target);
···
 
 			len += printk("%*s   %s", depth, "", usage_str[bit]);
 			len += printk(KERN_CONT " at:\n");
-			print_lock_trace(class->usage_traces + bit, len);
+			print_lock_trace(class->usage_traces[bit], len);
 		}
 	}
 	printk("%*s }\n", depth, "");
···
 	do {
 		print_lock_class_header(entry->class, depth);
 		printk("%*s ... acquired at:\n", depth, "");
-		print_lock_trace(&entry->trace, 2);
+		print_lock_trace(entry->trace, 2);
 		printk("\n");
 
 		if (depth == 0 && (entry != root)) {
···
 	print_lock_name(backwards_entry->class);
 	pr_warn("\n... which became %s-irq-safe at:\n", irqclass);
 
-	print_lock_trace(backwards_entry->class->usage_traces + bit1, 1);
+	print_lock_trace(backwards_entry->class->usage_traces[bit1], 1);
 
 	pr_warn("\nto a %s-irq-unsafe lock:\n", irqclass);
 	print_lock_name(forwards_entry->class);
 	pr_warn("\n... which became %s-irq-unsafe at:\n", irqclass);
 	pr_warn("...");
 
-	print_lock_trace(forwards_entry->class->usage_traces + bit2, 1);
+	print_lock_trace(forwards_entry->class->usage_traces[bit2], 1);
 
 	pr_warn("\nother info that might help us debug this:\n\n");
 	print_irq_lock_scenario(backwards_entry, forwards_entry,
···
 	lockdep_print_held_locks(curr);
 
 	pr_warn("\nthe dependencies between %s-irq-safe lock and the holding lock:\n", irqclass);
-	if (!save_trace(&prev_root->trace))
+	prev_root->trace = save_trace();
+	if (!prev_root->trace)
 		return;
 	print_shortest_lock_dependencies(backwards_entry, prev_root);
 
 	pr_warn("\nthe dependencies between the lock to be acquired");
 	pr_warn(" and %s-irq-unsafe lock:\n", irqclass);
-	if (!save_trace(&next_root->trace))
+	next_root->trace = save_trace();
+	if (!next_root->trace)
 		return;
 	print_shortest_lock_dependencies(forwards_entry, next_root);
 
···
  */
 static int
 check_prev_add(struct task_struct *curr, struct held_lock *prev,
-	       struct held_lock *next, int distance, struct lock_trace *trace)
+	       struct held_lock *next, int distance,
+	       struct lock_trace **const trace)
 {
 	struct lock_list *entry;
 	int ret;
···
 		return ret;
 #endif
 
-	if (!trace->nr_entries && !save_trace(trace))
-		return 0;
+	if (!*trace) {
+		*trace = save_trace();
+		if (!*trace)
+			return 0;
+	}
 
 	/*
 	 * Ok, all validations passed, add the new lock
···
 	 */
 	ret = add_lock_to_list(hlock_class(next), hlock_class(prev),
 			       &hlock_class(prev)->locks_after,
-			       next->acquire_ip, distance, trace);
+			       next->acquire_ip, distance, *trace);
 
 	if (!ret)
 		return 0;
 
 	ret = add_lock_to_list(hlock_class(prev), hlock_class(next),
 			       &hlock_class(next)->locks_before,
-			       next->acquire_ip, distance, trace);
+			       next->acquire_ip, distance, *trace);
 	if (!ret)
 		return 0;
 
···
 static int
 check_prevs_add(struct task_struct *curr, struct held_lock *next)
 {
-	struct lock_trace trace = { .nr_entries = 0 };
+	struct lock_trace *trace = NULL;
 	int depth = curr->lockdep_depth;
 	struct held_lock *hlock;
 
···
 	print_lock(this);
 
 	pr_warn("{%s} state was registered at:\n", usage_str[prev_bit]);
-	print_lock_trace(hlock_class(this)->usage_traces + prev_bit, 1);
+	print_lock_trace(hlock_class(this)->usage_traces[prev_bit], 1);
 
 	print_irqtrace_events(curr);
 	pr_warn("\nother info that might help us debug this:\n");
···
 	lockdep_print_held_locks(curr);
 
 	pr_warn("\nthe shortest dependencies between 2nd lock and 1st lock:\n");
-	if (!save_trace(&root->trace))
+	root->trace = save_trace();
+	if (!root->trace)
 		return;
 	print_shortest_lock_dependencies(other, root);
 
···
 
 	hlock_class(this)->usage_mask |= new_mask;
 
-	if (!save_trace(hlock_class(this)->usage_traces + new_bit))
+	if (!(hlock_class(this)->usage_traces[new_bit] = save_trace()))
 		return 0;
 
 	switch (new_bit) {
···
 #endif
 		) / 1024
 		);
+
+#if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_PROVE_LOCKING)
+	printk(" memory used for stack traces: %zu kB\n",
+	       (sizeof(stack_trace) + sizeof(stack_trace_hash)) / 1024
+	       );
+#endif
 
 	printk(" per task-struct memory footprint: %zu bytes\n",
 	       sizeof(((struct task_struct *)NULL)->held_locks));

kernel/locking/lockdep_internals.h

···
 #define MAX_LOCKDEP_ENTRIES	16384UL
 #define MAX_LOCKDEP_CHAINS_BITS	15
 #define MAX_STACK_TRACE_ENTRIES	262144UL
+#define STACK_TRACE_HASH_SIZE	8192
 #else
 #define MAX_LOCKDEP_ENTRIES	32768UL
 
···
  * addresses. Protected by the hash_lock.
  */
 #define MAX_STACK_TRACE_ENTRIES	524288UL
+#define STACK_TRACE_HASH_SIZE	16384
 #endif
 
 #define MAX_LOCKDEP_CHAINS	(1UL << MAX_LOCKDEP_CHAINS_BITS)
···
 extern void get_usage_chars(struct lock_class *class,
 			    char usage[LOCK_USAGE_CHARS]);
 
-extern const char * __get_key_name(struct lockdep_subclass_key *key, char *str);
+extern const char *__get_key_name(const struct lockdep_subclass_key *key,
+				  char *str);
 
 struct lock_class *lock_chain_get_class(struct lock_chain *chain, int i);
 
···
 #ifdef CONFIG_PROVE_LOCKING
 extern unsigned long lockdep_count_forward_deps(struct lock_class *);
 extern unsigned long lockdep_count_backward_deps(struct lock_class *);
+#ifdef CONFIG_TRACE_IRQFLAGS
+u64 lockdep_stack_trace_count(void);
+u64 lockdep_stack_hash_count(void);
+#endif
 #else
 static inline unsigned long
 lockdep_count_forward_deps(struct lock_class *class)

kernel/locking/lockdep_proc.c

···
 			nr_process_chains);
 	seq_printf(m, " stack-trace entries:           %11lu [max: %lu]\n",
 			nr_stack_trace_entries, MAX_STACK_TRACE_ENTRIES);
+#if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_PROVE_LOCKING)
+	seq_printf(m, " number of stack traces:        %llu\n",
+		   lockdep_stack_trace_count());
+	seq_printf(m, " number of stack hash chains:   %llu\n",
+		   lockdep_stack_hash_count());
+#endif
 	seq_printf(m, " combined max dependencies:     %11u\n",
 			(nr_hardirq_chains + 1) *
 			(nr_softirq_chains + 1) *
···
 
 static void seq_stats(struct seq_file *m, struct lock_stat_data *data)
 {
-	struct lockdep_subclass_key *ckey;
+	const struct lockdep_subclass_key *ckey;
 	struct lock_class_stats *stats;
 	struct lock_class *class;
 	const char *cname;

kernel/locking/mutex.c

···
 
 #define MUTEX_FLAGS		0x07
 
+/*
+ * Internal helper function; C doesn't allow us to hide it :/
+ *
+ * DO NOT USE (outside of mutex code).
+ */
+static inline struct task_struct *__mutex_owner(struct mutex *lock)
+{
+	return (struct task_struct *)(atomic_long_read(&lock->owner) & ~MUTEX_FLAGS);
+}
+
 static inline struct task_struct *__owner_task(unsigned long owner)
 {
 	return (struct task_struct *)(owner & ~MUTEX_FLAGS);
 }
+
+bool mutex_is_locked(struct mutex *lock)
+{
+	return __mutex_owner(lock) != NULL;
+}
+EXPORT_SYMBOL(mutex_is_locked);
+
+__must_check enum mutex_trylock_recursive_enum
+mutex_trylock_recursive(struct mutex *lock)
+{
+	if (unlikely(__mutex_owner(lock) == current))
+		return MUTEX_TRYLOCK_RECURSIVE;
+
+	return mutex_trylock(lock);
+}
+EXPORT_SYMBOL(mutex_trylock_recursive);
 
 static inline unsigned long __owner_flags(unsigned long owner)
 {

kernel/locking/rwsem.c

···
 #ifdef CONFIG_DEBUG_RWSEMS
 # define DEBUG_RWSEMS_WARN_ON(c, sem)	do {			\
 	if (!debug_locks_silent &&				\
-	    WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
+	    WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
 		#c, atomic_long_read(&(sem)->count),		\
+		(unsigned long) sem->magic,			\
 		atomic_long_read(&(sem)->owner), (long)current,	\
 		list_empty(&(sem)->wait_list) ? "" : "not "))	\
 			debug_locks_off();			\
···
 	 */
 	debug_check_no_locks_freed((void *)sem, sizeof(*sem));
 	lockdep_init_map(&sem->dep_map, name, key, 0);
+#endif
+#ifdef CONFIG_DEBUG_RWSEMS
+	sem->magic = sem;
 #endif
 	atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
 	raw_spin_lock_init(&sem->wait_lock);
···
 
 	rcu_read_lock();
 	for (;;) {
-		if (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF) {
-			state = OWNER_NONSPINNABLE;
-			break;
-		}
-
+		/*
+		 * When a waiting writer set the handoff flag, it may spin
+		 * on the owner as well. Once that writer acquires the lock,
+		 * we can spin on it. So we don't need to quit even when the
+		 * handoff bit is set.
+		 */
 		new = rwsem_owner_flags(sem, &new_flags);
 		if ((new != owner) || (new_flags != flags)) {
 			state = rwsem_owner_state(new, new_flags, nonspinnable);
···
 {
 	return false;
 }
+
+static inline int
+rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
+{
+	return 0;
+}
+#define OWNER_NULL	1
 #endif
 
 /*
···
 
 		raw_spin_unlock_irq(&sem->wait_lock);
 
+		/*
+		 * After setting the handoff bit and failing to acquire
+		 * the lock, attempt to spin on owner to accelerate lock
+		 * transfer. If the previous owner is a on-cpu writer and it
+		 * has just released the lock, OWNER_NULL will be returned.
+		 * In this case, we attempt to acquire the lock again
+		 * without sleeping.
+		 */
+		if ((wstate == WRITER_HANDOFF) &&
+		    (rwsem_spin_on_owner(sem, 0) == OWNER_NULL))
+			goto trylock_again;
+
 		/* Block until there are no active lockers. */
 		for (;;) {
 			if (signal_pending_state(state, current))
···
 				break;
 			}
 		}
-
+trylock_again:
 		raw_spin_lock_irq(&sem->wait_lock);
 	}
 	__set_current_state(TASK_RUNNING);
···
 
 static inline int __down_read_trylock(struct rw_semaphore *sem)
 {
+	long tmp;
+
+	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
+
 	/*
 	 * Optimize for the case when the rwsem is not locked at all.
 	 */
-	long tmp = RWSEM_UNLOCKED_VALUE;
-
+	tmp = RWSEM_UNLOCKED_VALUE;
 	do {
 		if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
 					tmp + RWSEM_READER_BIAS)) {
···
 
 static inline int __down_write_trylock(struct rw_semaphore *sem)
 {
-	long tmp = RWSEM_UNLOCKED_VALUE;
+	long tmp;
 
+	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
+
+	tmp  = RWSEM_UNLOCKED_VALUE;
 	if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
 					    RWSEM_WRITER_LOCKED)) {
 		rwsem_set_owner(sem);
···
 {
 	long tmp;
 
+	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, 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);
 	DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
···
 {
 	long tmp;
 
+	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, 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((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))

kernel/stacktrace.c

···
  * @nr_entries:	Number of entries in the storage array
  * @spaces:	Number of leading spaces to print
  */
-void stack_trace_print(unsigned long *entries, unsigned int nr_entries,
+void stack_trace_print(const unsigned long *entries, unsigned int nr_entries,
 		       int spaces)
 {
 	unsigned int i;
···
  *
  * Return: Number of bytes printed.
  */
-int stack_trace_snprint(char *buf, size_t size, unsigned long *entries,
+int stack_trace_snprint(char *buf, size_t size, const unsigned long *entries,
 			unsigned int nr_entries, int spaces)
 {
 	unsigned int generated, i, total = 0;