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

Pull locking and misc x86 updates from Ingo Molnar:
 "Lots of changes in this cycle - in part because locking/core attracted
  a number of related x86 low level work which was easier to handle in a
  single tree:

   - Linux Kernel Memory Consistency Model updates (Alan Stern, Paul E.
     McKenney, Andrea Parri)

   - lockdep scalability improvements and micro-optimizations (Waiman
     Long)

   - rwsem improvements (Waiman Long)

   - spinlock micro-optimization (Matthew Wilcox)

   - qspinlocks: Provide a liveness guarantee (more fairness) on x86.
     (Peter Zijlstra)

   - Add support for relative references in jump tables on arm64, x86
     and s390 to optimize jump labels (Ard Biesheuvel, Heiko Carstens)

   - Be a lot less permissive on weird (kernel address) uaccess faults
     on x86: BUG() when uaccess helpers fault on kernel addresses (Jann
     Horn)

   - macrofy x86 asm statements to un-confuse the GCC inliner. (Nadav
     Amit)

   - ... and a handful of other smaller changes as well"

* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (57 commits)
  locking/lockdep: Make global debug_locks* variables read-mostly
  locking/lockdep: Fix debug_locks off performance problem
  locking/pvqspinlock: Extend node size when pvqspinlock is configured
  locking/qspinlock_stat: Count instances of nested lock slowpaths
  locking/qspinlock, x86: Provide liveness guarantee
  x86/asm: 'Simplify' GEN_*_RMWcc() macros
  locking/qspinlock: Rework some comments
  locking/qspinlock: Re-order code
  locking/lockdep: Remove duplicated 'lock_class_ops' percpu array
  x86/defconfig: Enable CONFIG_USB_XHCI_HCD=y
  futex: Replace spin_is_locked() with lockdep
  locking/lockdep: Make class->ops a percpu counter and move it under CONFIG_DEBUG_LOCKDEP=y
  x86/jump-labels: Macrofy inline assembly code to work around GCC inlining bugs
  x86/cpufeature: Macrofy inline assembly code to work around GCC inlining bugs
  x86/extable: Macrofy inline assembly code to work around GCC inlining bugs
  x86/paravirt: Work around GCC inlining bugs when compiling paravirt ops
  x86/bug: Macrofy the BUG table section handling, to work around GCC inlining bugs
  x86/alternatives: Macrofy lock prefixes to work around GCC inlining bugs
  x86/refcount: Work around GCC inlining bug
  x86/objtool: Use asm macros to work around GCC inlining bugs
  ...

kernel/locking/lockdep.c

@@ -138,7 +138,7 @@ static struct lock_list list_entries[MAX_LOCKDEP_ENTRIES];
  * get freed - this significantly simplifies the debugging code.
  */
 unsigned long nr_lock_classes;
-static struct lock_class lock_classes[MAX_LOCKDEP_KEYS];
+struct lock_class lock_classes[MAX_LOCKDEP_KEYS];
 
 static inline struct lock_class *hlock_class(struct held_lock *hlock)
 {
@@ -1391,7 +1391,9 @@ static void print_lock_class_header(struct lock_class *class, int depth)
 
 	printk("%*s->", depth, "");
 	print_lock_name(class);
-	printk(KERN_CONT " ops: %lu", class->ops);
+#ifdef CONFIG_DEBUG_LOCKDEP
+	printk(KERN_CONT " ops: %lu", debug_class_ops_read(class));
+#endif
 	printk(KERN_CONT " {\n");
 
 	for (bit = 0; bit < LOCK_USAGE_STATES; bit++) {
@@ -2147,76 +2149,6 @@ static int check_no_collision(struct task_struct *curr,
 	return 1;
 }
 
-/*
- * This is for building a chain between just two different classes,
- * instead of adding a new hlock upon current, which is done by
- * add_chain_cache().
- *
- * This can be called in any context with two classes, while
- * add_chain_cache() must be done within the lock owener's context
- * since it uses hlock which might be racy in another context.
- */
-static inline int add_chain_cache_classes(unsigned int prev,
-					  unsigned int next,
-					  unsigned int irq_context,
-					  u64 chain_key)
-{
-	struct hlist_head *hash_head = chainhashentry(chain_key);
-	struct lock_chain *chain;
-
-	/*
-	 * Allocate a new chain entry from the static array, and add
-	 * it to the hash:
-	 */
-
-	/*
-	 * We might need to take the graph lock, ensure we've got IRQs
-	 * disabled to make this an IRQ-safe lock.. for recursion reasons
-	 * lockdep won't complain about its own locking errors.
-	 */
-	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
-		return 0;
-
-	if (unlikely(nr_lock_chains >= MAX_LOCKDEP_CHAINS)) {
-		if (!debug_locks_off_graph_unlock())
-			return 0;
-
-		print_lockdep_off("BUG: MAX_LOCKDEP_CHAINS too low!");
-		dump_stack();
-		return 0;
-	}
-
-	chain = lock_chains + nr_lock_chains++;
-	chain->chain_key = chain_key;
-	chain->irq_context = irq_context;
-	chain->depth = 2;
-	if (likely(nr_chain_hlocks + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
-		chain->base = nr_chain_hlocks;
-		nr_chain_hlocks += chain->depth;
-		chain_hlocks[chain->base] = prev - 1;
-		chain_hlocks[chain->base + 1] = next -1;
-	}
-#ifdef CONFIG_DEBUG_LOCKDEP
-	/*
-	 * Important for check_no_collision().
-	 */
-	else {
-		if (!debug_locks_off_graph_unlock())
-			return 0;
-
-		print_lockdep_off("BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!");
-		dump_stack();
-		return 0;
-	}
-#endif
-
-	hlist_add_head_rcu(&chain->entry, hash_head);
-	debug_atomic_inc(chain_lookup_misses);
-	inc_chains();
-
-	return 1;
-}
-
 /*
  * Adds a dependency chain into chain hashtable. And must be called with
  * graph_lock held.
@@ -3262,6 +3194,10 @@ static int __lock_is_held(const struct lockdep_map *lock, int read);
 /*
  * This gets called for every mutex_lock*()/spin_lock*() operation.
  * We maintain the dependency maps and validate the locking attempt:
+ *
+ * The callers must make sure that IRQs are disabled before calling it,
+ * otherwise we could get an interrupt which would want to take locks,
+ * which would end up in lockdep again.
  */
 static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 			  int trylock, int read, int check, int hardirqs_off,
@@ -3279,14 +3215,6 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	if (unlikely(!debug_locks))
 		return 0;
 
-	/*
-	 * Lockdep should run with IRQs disabled, otherwise we could
-	 * get an interrupt which would want to take locks, which would
-	 * end up in lockdep and have you got a head-ache already?
-	 */
-	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
-		return 0;
-
 	if (!prove_locking || lock->key == &__lockdep_no_validate__)
 		check = 0;
 
@@ -3300,7 +3228,9 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 		if (!class)
 			return 0;
 	}
-	atomic_inc((atomic_t *)&class->ops);
+
+	debug_class_ops_inc(class);
+
 	if (very_verbose(class)) {
 		printk("\nacquire class [%px] %s", class->key, class->name);
 		if (class->name_version > 1)
@@ -3543,6 +3473,9 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
 {
 	struct held_lock *hlock;
 
+	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
+		return 0;
+
 	for (hlock = curr->held_locks + idx; idx < depth; idx++, hlock++) {
 		if (!__lock_acquire(hlock->instance,
 				    hlock_class(hlock)->subclass,
@@ -3696,6 +3629,13 @@ __lock_release(struct lockdep_map *lock, int nested, unsigned long ip)
 	curr->lockdep_depth = i;
 	curr->curr_chain_key = hlock->prev_chain_key;
 
+	/*
+	 * The most likely case is when the unlock is on the innermost
+	 * lock. In this case, we are done!
+	 */
+	if (i == depth-1)
+		return 1;
+
 	if (reacquire_held_locks(curr, depth, i + 1))
 		return 0;
 
@@ -3703,10 +3643,14 @@ __lock_release(struct lockdep_map *lock, int nested, unsigned long ip)
 	 * We had N bottles of beer on the wall, we drank one, but now
 	 * there's not N-1 bottles of beer left on the wall...
 	 */
-	if (DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth - 1))
-		return 0;
+	DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth-1);
 
-	return 1;
+	/*
+	 * Since reacquire_held_locks() would have called check_chain_key()
+	 * indirectly via __lock_acquire(), we don't need to do it again
+	 * on return.
+	 */
+	return 0;
 }
 
 static int __lock_is_held(const struct lockdep_map *lock, int read)
@@ -4122,7 +4066,7 @@ void lock_contended(struct lockdep_map *lock, unsigned long ip)
 {
 	unsigned long flags;
 
-	if (unlikely(!lock_stat))
+	if (unlikely(!lock_stat || !debug_locks))
 		return;
 
 	if (unlikely(current->lockdep_recursion))
@@ -4142,7 +4086,7 @@ void lock_acquired(struct lockdep_map *lock, unsigned long ip)
 {
 	unsigned long flags;
 
-	if (unlikely(!lock_stat))
+	if (unlikely(!lock_stat || !debug_locks))
 		return;
 
 	if (unlikely(current->lockdep_recursion))

kernel/locking/lockdep_internals.h

@@ -152,9 +152,15 @@ struct lockdep_stats {
 	int	nr_find_usage_forwards_recursions;
 	int	nr_find_usage_backwards_checks;
 	int	nr_find_usage_backwards_recursions;
+
+	/*
+	 * Per lock class locking operation stat counts
+	 */
+	unsigned long lock_class_ops[MAX_LOCKDEP_KEYS];
 };
 
 DECLARE_PER_CPU(struct lockdep_stats, lockdep_stats);
+extern struct lock_class lock_classes[MAX_LOCKDEP_KEYS];
 
 #define __debug_atomic_inc(ptr)					\
 	this_cpu_inc(lockdep_stats.ptr);
@@ -179,9 +185,30 @@ DECLARE_PER_CPU(struct lockdep_stats, lockdep_stats);
 	}								\
 	__total;							\
 })
+
+static inline void debug_class_ops_inc(struct lock_class *class)
+{
+	int idx;
+
+	idx = class - lock_classes;
+	__debug_atomic_inc(lock_class_ops[idx]);
+}
+
+static inline unsigned long debug_class_ops_read(struct lock_class *class)
+{
+	int idx, cpu;
+	unsigned long ops = 0;
+
+	idx = class - lock_classes;
+	for_each_possible_cpu(cpu)
+		ops += per_cpu(lockdep_stats.lock_class_ops[idx], cpu);
+	return ops;
+}
+
 #else
 # define __debug_atomic_inc(ptr)	do { } while (0)
 # define debug_atomic_inc(ptr)		do { } while (0)
 # define debug_atomic_dec(ptr)		do { } while (0)
 # define debug_atomic_read(ptr)		0
+# define debug_class_ops_inc(ptr)	do { } while (0)
 #endif

kernel/locking/lockdep_proc.c

@@ -68,7 +68,7 @@ static int l_show(struct seq_file *m, void *v)
 
 	seq_printf(m, "%p", class->key);
 #ifdef CONFIG_DEBUG_LOCKDEP
-	seq_printf(m, " OPS:%8ld", class->ops);
+	seq_printf(m, " OPS:%8ld", debug_class_ops_read(class));
 #endif
 #ifdef CONFIG_PROVE_LOCKING
 	seq_printf(m, " FD:%5ld", lockdep_count_forward_deps(class));

kernel/locking/qspinlock.c

@@ -74,12 +74,24 @@
  */
 
 #include "mcs_spinlock.h"
-
-#ifdef CONFIG_PARAVIRT_SPINLOCKS
-#define MAX_NODES	8
-#else
 #define MAX_NODES	4
+
+/*
+ * On 64-bit architectures, the mcs_spinlock structure will be 16 bytes in
+ * size and four of them will fit nicely in one 64-byte cacheline. For
+ * pvqspinlock, however, we need more space for extra data. To accommodate
+ * that, we insert two more long words to pad it up to 32 bytes. IOW, only
+ * two of them can fit in a cacheline in this case. That is OK as it is rare
+ * to have more than 2 levels of slowpath nesting in actual use. We don't
+ * want to penalize pvqspinlocks to optimize for a rare case in native
+ * qspinlocks.
+ */
+struct qnode {
+	struct mcs_spinlock mcs;
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+	long reserved[2];
 #endif
+};
 
 /*
  * The pending bit spinning loop count.
@@ -101,7 +113,7 @@
  *
  * PV doubles the storage and uses the second cacheline for PV state.
  */
-static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]);
+static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]);
 
 /*
  * We must be able to distinguish between no-tail and the tail at 0:0,
@@ -126,7 +138,13 @@ 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;
 
-	return per_cpu_ptr(&mcs_nodes[idx], cpu);
+	return per_cpu_ptr(&qnodes[idx].mcs, cpu);
+}
+
+static inline __pure
+struct mcs_spinlock *grab_mcs_node(struct mcs_spinlock *base, int idx)
+{
+	return &((struct qnode *)base + idx)->mcs;
 }
 
 #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
@@ -231,6 +249,20 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
 }
 #endif /* _Q_PENDING_BITS == 8 */
 
+/**
+ * queued_fetch_set_pending_acquire - fetch the whole lock value and set pending
+ * @lock : Pointer to queued spinlock structure
+ * Return: The previous lock value
+ *
+ * *,*,* -> *,1,*
+ */
+#ifndef queued_fetch_set_pending_acquire
+static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lock)
+{
+	return atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
+}
+#endif
+
 /**
  * set_locked - Set the lock bit and own the lock
  * @lock: Pointer to queued spinlock structure
@@ -326,43 +358,48 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	/*
 	 * trylock || pending
 	 *
-	 * 0,0,0 -> 0,0,1 ; trylock
-	 * 0,0,1 -> 0,1,1 ; pending
+	 * 0,0,* -> 0,1,* -> 0,0,1 pending, trylock
 	 */
-	val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
-	if (!(val & ~_Q_LOCKED_MASK)) {
-		/*
-		 * We're pending, wait for the owner to go away.
-		 *
-		 * *,1,1 -> *,1,0
-		 *
-		 * this wait loop must be a load-acquire such that we match the
-		 * store-release that clears the locked bit and create lock
-		 * sequentiality; this is because not all
-		 * clear_pending_set_locked() implementations imply full
-		 * barriers.
-		 */
-		if (val & _Q_LOCKED_MASK) {
-			atomic_cond_read_acquire(&lock->val,
-						 !(VAL & _Q_LOCKED_MASK));
-		}
+	val = queued_fetch_set_pending_acquire(lock);
 
-		/*
-		 * take ownership and clear the pending bit.
-		 *
-		 * *,1,0 -> *,0,1
-		 */
-		clear_pending_set_locked(lock);
-		qstat_inc(qstat_lock_pending, true);
-		return;
+	/*
+	 * If we observe contention, there is a concurrent locker.
+	 *
+	 * Undo and queue; our setting of PENDING might have made the
+	 * n,0,0 -> 0,0,0 transition fail and it will now be waiting
+	 * on @next to become !NULL.
+	 */
+	if (unlikely(val & ~_Q_LOCKED_MASK)) {
+
+		/* Undo PENDING if we set it. */
+		if (!(val & _Q_PENDING_MASK))
+			clear_pending(lock);
+
+		goto queue;
 	}
 
 	/*
-	 * If pending was clear but there are waiters in the queue, then
-	 * we need to undo our setting of pending before we queue ourselves.
+	 * We're pending, wait for the owner to go away.
+	 *
+	 * 0,1,1 -> 0,1,0
+	 *
+	 * this wait loop must be a load-acquire such that we match the
+	 * store-release that clears the locked bit and create lock
+	 * sequentiality; this is because not all
+	 * clear_pending_set_locked() implementations imply full
+	 * barriers.
 	 */
-	if (!(val & _Q_PENDING_MASK))
-		clear_pending(lock);
+	if (val & _Q_LOCKED_MASK)
+		atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_MASK));
+
+	/*
+	 * take ownership and clear the pending bit.
+	 *
+	 * 0,1,0 -> 0,0,1
+	 */
+	clear_pending_set_locked(lock);
+	qstat_inc(qstat_lock_pending, true);
+	return;
 
 	/*
 	 * End of pending bit optimistic spinning and beginning of MCS
@@ -371,11 +408,16 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 queue:
 	qstat_inc(qstat_lock_slowpath, true);
 pv_queue:
-	node = this_cpu_ptr(&mcs_nodes[0]);
+	node = this_cpu_ptr(&qnodes[0].mcs);
 	idx = node->count++;
 	tail = encode_tail(smp_processor_id(), idx);
 
-	node += idx;
+	node = grab_mcs_node(node, idx);
+
+	/*
+	 * Keep counts of non-zero index values:
+	 */
+	qstat_inc(qstat_lock_idx1 + idx - 1, idx);
 
 	/*
 	 * Ensure that we increment the head node->count before initialising
@@ -476,16 +518,25 @@ locked:
 	 */
 
 	/*
-	 * In the PV case we might already have _Q_LOCKED_VAL set.
+	 * In the PV case we might already have _Q_LOCKED_VAL set, because
+	 * of lock stealing; therefore we must also allow:
 	 *
-	 * The atomic_cond_read_acquire() call above has provided the
-	 * necessary acquire semantics required for locking.
+	 * n,0,1 -> 0,0,1
+	 *
+	 * Note: at this point: (val & _Q_PENDING_MASK) == 0, because of the
+	 *       above wait condition, therefore any concurrent setting of
+	 *       PENDING will make the uncontended transition fail.
 	 */
-	if (((val & _Q_TAIL_MASK) == tail) &&
-	    atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
-		goto release; /* No contention */
+	if ((val & _Q_TAIL_MASK) == tail) {
+		if (atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
+			goto release; /* No contention */
+	}
 
-	/* Either somebody is queued behind us or _Q_PENDING_VAL is set */
+	/*
+	 * Either somebody is queued behind us or _Q_PENDING_VAL got set
+	 * which will then detect the remaining tail and queue behind us
+	 * ensuring we'll see a @next.
+	 */
 	set_locked(lock);
 
 	/*
@@ -501,7 +552,7 @@ release:
 	/*
 	 * release the node
 	 */
-	__this_cpu_dec(mcs_nodes[0].count);
+	__this_cpu_dec(qnodes[0].mcs.count);
 }
 EXPORT_SYMBOL(queued_spin_lock_slowpath);
 

kernel/locking/qspinlock_paravirt.h

@@ -49,8 +49,6 @@ enum vcpu_state {
 
 struct pv_node {
 	struct mcs_spinlock	mcs;
-	struct mcs_spinlock	__res[3];
-
 	int			cpu;
 	u8			state;
 };
@@ -281,7 +279,7 @@ static void pv_init_node(struct mcs_spinlock *node)
 {
 	struct pv_node *pn = (struct pv_node *)node;
 
-	BUILD_BUG_ON(sizeof(struct pv_node) > 5*sizeof(struct mcs_spinlock));
+	BUILD_BUG_ON(sizeof(struct pv_node) > sizeof(struct qnode));
 
 	pn->cpu = smp_processor_id();
 	pn->state = vcpu_running;

kernel/locking/qspinlock_stat.h

@@ -55,6 +55,9 @@ enum qlock_stats {
 	qstat_pv_wait_node,
 	qstat_lock_pending,
 	qstat_lock_slowpath,
+	qstat_lock_idx1,
+	qstat_lock_idx2,
+	qstat_lock_idx3,
 	qstat_num,	/* Total number of statistical counters */
 	qstat_reset_cnts = qstat_num,
 };
@@ -82,6 +85,9 @@ static const char * const qstat_names[qstat_num + 1] = {
 	[qstat_pv_wait_node]       = "pv_wait_node",
 	[qstat_lock_pending]       = "lock_pending",
 	[qstat_lock_slowpath]      = "lock_slowpath",
+	[qstat_lock_idx1]	   = "lock_index1",
+	[qstat_lock_idx2]	   = "lock_index2",
+	[qstat_lock_idx3]	   = "lock_index3",
 	[qstat_reset_cnts]         = "reset_counters",
 };
 

kernel/locking/rtmutex.c

@@ -1485,9 +1485,9 @@ void __sched rt_mutex_lock_nested(struct rt_mutex *lock, unsigned int subclass)
 	__rt_mutex_lock(lock, subclass);
 }
 EXPORT_SYMBOL_GPL(rt_mutex_lock_nested);
-#endif
 
-#ifndef CONFIG_DEBUG_LOCK_ALLOC
+#else /* !CONFIG_DEBUG_LOCK_ALLOC */
+
 /**
  * rt_mutex_lock - lock a rt_mutex
  *

kernel/locking/rwsem-xadd.c

@@ -180,7 +180,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
 		 * but it gives the spinners an early indication that the
 		 * readers now have the lock.
 		 */
-		rwsem_set_reader_owned(sem);
+		__rwsem_set_reader_owned(sem, waiter->task);
 	}
 
 	/*
@@ -233,8 +233,19 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
 	waiter.type = RWSEM_WAITING_FOR_READ;
 
 	raw_spin_lock_irq(&sem->wait_lock);
-	if (list_empty(&sem->wait_list))
+	if (list_empty(&sem->wait_list)) {
+		/*
+		 * In case the wait queue is empty and the lock isn't owned
+		 * by a writer, this reader can exit the slowpath and return
+		 * immediately as its RWSEM_ACTIVE_READ_BIAS has already
+		 * been set in the count.
+		 */
+		if (atomic_long_read(&sem->count) >= 0) {
+			raw_spin_unlock_irq(&sem->wait_lock);
+			return sem;
+		}
 		adjustment += RWSEM_WAITING_BIAS;
+	}
 	list_add_tail(&waiter.list, &sem->wait_list);
 
 	/* we're now waiting on the lock, but no longer actively locking */

kernel/locking/rwsem.c

@@ -117,8 +117,9 @@ EXPORT_SYMBOL(down_write_trylock);
 void up_read(struct rw_semaphore *sem)
 {
 	rwsem_release(&sem->dep_map, 1, _RET_IP_);
-	DEBUG_RWSEMS_WARN_ON(sem->owner != RWSEM_READER_OWNED);
+	DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED));
 
+	rwsem_clear_reader_owned(sem);
 	__up_read(sem);
 }
 
@@ -181,7 +182,7 @@ void down_read_non_owner(struct rw_semaphore *sem)
 	might_sleep();
 
 	__down_read(sem);
-	rwsem_set_reader_owned(sem);
+	__rwsem_set_reader_owned(sem, NULL);
 }
 
 EXPORT_SYMBOL(down_read_non_owner);
@@ -215,7 +216,7 @@ EXPORT_SYMBOL(down_write_killable_nested);
 
 void up_read_non_owner(struct rw_semaphore *sem)
 {
-	DEBUG_RWSEMS_WARN_ON(sem->owner != RWSEM_READER_OWNED);
+	DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED));
 	__up_read(sem);
 }
 

kernel/locking/rwsem.h

@@ -1,24 +1,30 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /*
- * The owner field of the rw_semaphore structure will be set to
- * RWSEM_READER_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.
+ * The least significant 2 bits of the owner value has the following
+ * meanings when set.
+ *  - RWSEM_READER_OWNED (bit 0): The rwsem is owned by readers
+ *  - RWSEM_ANONYMOUSLY_OWNED (bit 1): The rwsem is anonymously owned,
+ *    i.e. the owner(s) cannot be readily determined. It can be reader
+ *    owned or the owning writer is indeterminate.
  *
- * In essence, the owner field now has the following 4 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) RWSEM_ANONYMOUSLY_OWNED bit set with some other bits set as well
- *     - lock is owned by an anonymous writer, so spinning on the lock
- *       owner should be disabled.
- *  4) Other non-zero value
- *     - a writer owns the lock and other writers can spin on the lock owner.
+ * When a writer acquires a rwsem, it puts its task_struct pointer
+ * into the owner field. It is cleared after an unlock.
+ *
+ * When a reader acquires a rwsem, it will also puts its task_struct
+ * pointer into the owner field with both the RWSEM_READER_OWNED and
+ * RWSEM_ANONYMOUSLY_OWNED bits set. On unlock, the owner field will
+ * largely be left untouched. So for a free or reader-owned rwsem,
+ * the owner value may contain information about the last reader that
+ * acquires the rwsem. The anonymous bit is set because that particular
+ * reader may or may not still own the lock.
+ *
+ * That information may be helpful in debugging cases where the system
+ * seems to hang on a reader owned rwsem especially if only one reader
+ * is involved. Ideally we would like to track all the readers that own
+ * a rwsem, but the overhead is simply too big.
  */
-#define RWSEM_ANONYMOUSLY_OWNED	(1UL << 0)
-#define RWSEM_READER_OWNED	((struct task_struct *)RWSEM_ANONYMOUSLY_OWNED)
+#define RWSEM_READER_OWNED	(1UL << 0)
+#define RWSEM_ANONYMOUSLY_OWNED	(1UL << 1)
 
 #ifdef CONFIG_DEBUG_RWSEMS
 # define DEBUG_RWSEMS_WARN_ON(c)	DEBUG_LOCKS_WARN_ON(c)
@@ -44,15 +50,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 	WRITE_ONCE(sem->owner, NULL);
 }
 
+/*
+ * The task_struct pointer of the last owning reader will be left in
+ * the owner field.
+ *
+ * Note that the owner value just indicates the task has owned the rwsem
+ * previously, it may not be the real owner or one of the real owners
+ * anymore when that field is examined, so take it with a grain of salt.
+ */
+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_ANONYMOUSLY_OWNED;
+
+	WRITE_ONCE(sem->owner, (struct task_struct *)val);
+}
+
 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 (READ_ONCE(sem->owner) != RWSEM_READER_OWNED)
-		WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
+	__rwsem_set_reader_owned(sem, current);
 }
 
 /*
@@ -72,6 +89,25 @@ static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
 {
 	return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED;
 }
+
+#ifdef CONFIG_DEBUG_RWSEMS
+/*
+ * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
+ * is a task pointer in owner of a reader-owned rwsem, it will be the
+ * real owner or one of the real owners. The only exception is when the
+ * unlock is done by up_read_non_owner().
+ */
+#define rwsem_clear_reader_owned rwsem_clear_reader_owned
+static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
+{
+	unsigned long val = (unsigned long)current | RWSEM_READER_OWNED
+						   | RWSEM_ANONYMOUSLY_OWNED;
+	if (READ_ONCE(sem->owner) == (struct task_struct *)val)
+		cmpxchg_relaxed((unsigned long *)&sem->owner, val,
+				RWSEM_READER_OWNED | RWSEM_ANONYMOUSLY_OWNED);
+}
+#endif
+
 #else
 static inline void rwsem_set_owner(struct rw_semaphore *sem)
 {
@@ -81,7 +117,18 @@ 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)
+{
+}
+
 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
 {
 }
 #endif
+
+#ifndef rwsem_clear_reader_owned
+static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
+{
+}
+#endif