[PATCH 4/3] locking/lockdep: Improve the deadlock scenario print for sync and read lock

[Boqun Feng <boqun.feng@gmail.com>]

Lock scenario print is always a weak spot of lockdep splats. Improvement
can be made if we rework the dependency search and the error printing.

However without touching the graph search, we can improve a little for
the circular deadlock case, since we have the to-be-added lock
dependency, and know whether these two locks are read/write/sync.

In order to know whether a held_lock is sync or not, a bit was
"stolen" from ->references, which reduce our limit for the same lock
class nesting from 2^12 to 2^11, and it should still be good enough.

Besides, since we now have bit in held_lock for sync, we don't need the
"hardirqoffs being 1" trick, and also we can avoid the __lock_release()
if we jump out of __lock_acquire() before the held_lock stored.

With these changes, a deadlock case evolved with read lock and sync gets
a better print-out from:

	[...]  Possible unsafe locking scenario:
	[...]
	[...]        CPU0                    CPU1
	[...]        ----                    ----
	[...]   lock(srcuA);
	[...]                                lock(srcuB);
	[...]                                lock(srcuA);
	[...]   lock(srcuB);

to

	[...]  Possible unsafe locking scenario:
	[...]
	[...]        CPU0                    CPU1
	[...]        ----                    ----
	[...]   rlock(srcuA);
	[...]                                lock(srcuB);
	[...]                                lock(srcuA);
	[...]   sync(srcuB);

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 include/linux/lockdep.h  |  3 ++-
 kernel/locking/lockdep.c | 48 ++++++++++++++++++++++++++--------------
 2 files changed, 34 insertions(+), 17 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index ba09df6a0872..febd7ecc225c 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -134,7 +134,8 @@ struct held_lock {
 	unsigned int read:2;        /* see lock_acquire() comment */
 	unsigned int check:1;       /* see lock_acquire() comment */
 	unsigned int hardirqs_off:1;
-	unsigned int references:12;					/* 32 bits */
+	unsigned int sync:1;
+	unsigned int references:11;					/* 32 bits */
 	unsigned int pin_count;
 };
 
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index cffa026a765f..4031d87f6829 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1880,6 +1880,8 @@ print_circular_lock_scenario(struct held_lock *src,
 	struct lock_class *source = hlock_class(src);
 	struct lock_class *target = hlock_class(tgt);
 	struct lock_class *parent = prt->class;
+	int src_read = src->read;
+	int tgt_read = tgt->read;
 
 	/*
 	 * A direct locking problem where unsafe_class lock is taken
@@ -1907,7 +1909,10 @@ print_circular_lock_scenario(struct held_lock *src,
 	printk(" Possible unsafe locking scenario:\n\n");
 	printk("       CPU0                    CPU1\n");
 	printk("       ----                    ----\n");
-	printk("  lock(");
+	if (tgt_read != 0)
+		printk("  rlock(");
+	else
+		printk("  lock(");
 	__print_lock_name(target);
 	printk(KERN_CONT ");\n");
 	printk("                               lock(");
@@ -1916,7 +1921,12 @@ print_circular_lock_scenario(struct held_lock *src,
 	printk("                               lock(");
 	__print_lock_name(target);
 	printk(KERN_CONT ");\n");
-	printk("  lock(");
+	if (src_read != 0)
+		printk("  rlock(");
+	else if (src->sync)
+		printk("  sync(");
+	else
+		printk("  lock(");
 	__print_lock_name(source);
 	printk(KERN_CONT ");\n");
 	printk("\n *** DEADLOCK ***\n\n");
@@ -4530,7 +4540,13 @@ mark_usage(struct task_struct *curr, struct held_lock *hlock, int check)
 					return 0;
 		}
 	}
-	if (!hlock->hardirqs_off) {
+
+	/*
+	 * For lock_sync(), don't mark the ENABLED usage, since lock_sync()
+	 * creates no critical section and no extra dependency can be introduced
+	 * by interrupts
+	 */
+	if (!hlock->hardirqs_off && !hlock->sync) {
 		if (hlock->read) {
 			if (!mark_lock(curr, hlock,
 					LOCK_ENABLED_HARDIRQ_READ))
@@ -4909,7 +4925,7 @@ static int __lock_is_held(const struct lockdep_map *lock, int read);
 static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 			  int trylock, int read, int check, int hardirqs_off,
 			  struct lockdep_map *nest_lock, unsigned long ip,
-			  int references, int pin_count)
+			  int references, int pin_count, int sync)
 {
 	struct task_struct *curr = current;
 	struct lock_class *class = NULL;
@@ -4960,7 +4976,8 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 	class_idx = class - lock_classes;
 
-	if (depth) { /* we're holding locks */
+	if (depth && !sync) {
+		/* we're holding locks and the new held lock is not a sync */
 		hlock = curr->held_locks + depth - 1;
 		if (hlock->class_idx == class_idx && nest_lock) {
 			if (!references)
@@ -4994,6 +5011,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	hlock->trylock = trylock;
 	hlock->read = read;
 	hlock->check = check;
+	hlock->sync = !!sync;
 	hlock->hardirqs_off = !!hardirqs_off;
 	hlock->references = references;
 #ifdef CONFIG_LOCK_STAT
@@ -5055,6 +5073,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	if (!validate_chain(curr, hlock, chain_head, chain_key))
 		return 0;
 
+	/* For lock_sync(), we are done here since no actual critical section */
+	if (hlock->sync)
+		return 1;
+
 	curr->curr_chain_key = chain_key;
 	curr->lockdep_depth++;
 	check_chain_key(curr);
@@ -5196,7 +5218,7 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
 				    hlock->read, hlock->check,
 				    hlock->hardirqs_off,
 				    hlock->nest_lock, hlock->acquire_ip,
-				    hlock->references, hlock->pin_count)) {
+				    hlock->references, hlock->pin_count, 0)) {
 		case 0:
 			return 1;
 		case 1:
@@ -5666,7 +5688,7 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 	lockdep_recursion_inc();
 	__lock_acquire(lock, subclass, trylock, read, check,
-		       irqs_disabled_flags(flags), nest_lock, ip, 0, 0);
+		       irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 0);
 	lockdep_recursion_finish();
 	raw_local_irq_restore(flags);
 }
@@ -5699,11 +5721,6 @@ EXPORT_SYMBOL_GPL(lock_release);
  * APIs are used to wait for one or multiple critical sections (on other CPUs
  * or threads), and it means that calling these APIs inside these critical
  * sections is potential deadlock.
- *
- * This annotation acts as an acqurie+release anontation pair with hardirqoff
- * being 1. Since there's no critical section, no interrupt can create extra
- * dependencies "inside" the annotation, hardirqoff == 1 allows us to avoid
- * false positives.
  */
 void lock_sync(struct lockdep_map *lock, unsigned subclass, int read,
 	       int check, struct lockdep_map *nest_lock, unsigned long ip)
@@ -5717,10 +5734,9 @@ void lock_sync(struct lockdep_map *lock, unsigned subclass, int read,
 	check_flags(flags);
 
 	lockdep_recursion_inc();
-	__lock_acquire(lock, subclass, 0, read, check, 1, nest_lock, ip, 0, 0);
-
-	if (__lock_release(lock, ip))
-		check_chain_key(current);
+	__lock_acquire(lock, subclass, 0, read, check,
+		       irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 1);
+	check_chain_key(current);
 	lockdep_recursion_finish();
 	raw_local_irq_restore(flags);
 }
-- 
2.38.1