[PATCH v3 00/18] locking/lockdep: Add comments and make some code

[PATCH v3 00/18] locking/lockdep: Add comments and make some code

[Yuyang Du]

Hi Peter,

I recently looked at some system hang issues. While at it, I tried to use
and understand lockdep. These patches are made as a result. I believe they
should have helped me, so hopefully they do for others as well.

Many thanks to Bart, Joe, and Peter for their valuable comments.

Change from v2:
- Removed indent adjustments only patch
- Removed unnecessary if to else-if patch
- Made changes according to comments
- Added another doc addition patch

Change from v1:
- Rebased the patch series.
- Added more no-functional-change patches.
- Removed zapped locks in lock chains printing patch, which was a band-aid.
  The real problem was recently fixed by Bart.

Thanks,
Yuyang

--

Yuyang Du (18):
  locking/lockdep: Change all print_*() return type to void
  locking/lockdep: Add description and explanation in lockdep design doc
  locking/lockdep: Adjust lock usage bit character checks
  locking/lockdep: Remove useless conditional macro
  locking/lockdep: Print the right depth for chain key colission
  locking/lockdep: Update obsolete struct field description
  locking/lockdep: Use lockdep_init_task for task initiation
    consistently
  locking/lockdep: Define INITIAL_CHAIN_KEY for chain keys to start with
  locking/lockdep: Change the range of class_idx in held_lock struct
  locking/lockdep: Remove unused argument in validate_chain() and    
    check_deadlock()
  locking/lockdep: Update comment
  locking/lockdep: Remove unnecessary function pointer argument
  locking/lockdep: Change type of the element field in circular_queue
  locking/lockdep: Change the return type of __cq_dequeue()
  locking/lockdep: Avoid constant checks in __bfs by using offset
    reference
  locking/lockdep: Combine check_noncircular and check_redundant
  locking/lockdep: Update comments on dependency search
  locking/lockdep: Add explanation to lock usage rules in lockdep design
    doc

 Documentation/locking/lockdep-design.txt | 115 ++++++--
 include/linux/lockdep.h                  |  30 +-
 init/init_task.c                         |   2 +
 kernel/fork.c                            |   3 -
 kernel/locking/lockdep.c                 | 487 +++++++++++++++++--------------
 kernel/locking/lockdep_internals.h       |   1 +
 6 files changed, 378 insertions(+), 260 deletions(-)

-- 
1.8.3.1

[PATCH v3 01/18] locking/lockdep: Change all print_*() return type to void

[Yuyang Du]

Since none of the print_*() function's return value is necessary, change
their return type to void. No functional change.

In cases where an invariable return value is used, this change slightly
improves readability, i.e.:

	print_x();
	return 0;

is definitely better than:

	return print_x(); /* where print_x() always returns 0 */

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 203 ++++++++++++++++++++++++-----------------------
 1 file changed, 103 insertions(+), 100 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 34cdcbe..41137ad 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1430,17 +1430,15 @@ static inline int __bfs_backwards(struct lock_list *src_entry,
  * Print a dependency chain entry (this is only done when a deadlock
  * has been detected):
  */
-static noinline int
+static noinline void
 print_circular_bug_entry(struct lock_list *target, int depth)
 {
 	if (debug_locks_silent)
-		return 0;
+		return;
 	printk("\n-> #%u", depth);
 	print_lock_name(target->class);
 	printk(KERN_CONT ":\n");
 	print_stack_trace(&target->trace, 6);
-
-	return 0;
 }
 
 static void
@@ -1497,7 +1495,7 @@ static inline int __bfs_backwards(struct lock_list *src_entry,
  * When a circular dependency is detected, print the
  * header first:
  */
-static noinline int
+static noinline void
 print_circular_bug_header(struct lock_list *entry, unsigned int depth,
 			struct held_lock *check_src,
 			struct held_lock *check_tgt)
@@ -1505,7 +1503,7 @@ static inline int __bfs_backwards(struct lock_list *src_entry,
 	struct task_struct *curr = current;
 
 	if (debug_locks_silent)
-		return 0;
+		return;
 
 	pr_warn("\n");
 	pr_warn("======================================================\n");
@@ -1523,8 +1521,6 @@ static inline int __bfs_backwards(struct lock_list *src_entry,
 	pr_warn("\nthe existing dependency chain (in reverse order) is:\n");
 
 	print_circular_bug_entry(entry, depth);
-
-	return 0;
 }
 
 static inline int class_equal(struct lock_list *entry, void *data)
@@ -1532,11 +1528,11 @@ static inline int class_equal(struct lock_list *entry, void *data)
 	return entry->class == data;
 }
 
-static noinline int print_circular_bug(struct lock_list *this,
-				struct lock_list *target,
-				struct held_lock *check_src,
-				struct held_lock *check_tgt,
-				struct stack_trace *trace)
+static noinline void print_circular_bug(struct lock_list *this,
+					struct lock_list *target,
+					struct held_lock *check_src,
+					struct held_lock *check_tgt,
+					struct stack_trace *trace)
 {
 	struct task_struct *curr = current;
 	struct lock_list *parent;
@@ -1544,10 +1540,10 @@ static noinline int print_circular_bug(struct lock_list *this,
 	int depth;
 
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
-		return 0;
+		return;
 
 	if (!save_trace(&this->trace))
-		return 0;
+		return;
 
 	depth = get_lock_depth(target);
 
@@ -1569,21 +1565,17 @@ static noinline int print_circular_bug(struct lock_list *this,
 
 	printk("\nstack backtrace:\n");
 	dump_stack();
-
-	return 0;
 }
 
-static noinline int print_bfs_bug(int ret)
+static noinline void print_bfs_bug(int ret)
 {
 	if (!debug_locks_off_graph_unlock())
-		return 0;
+		return;
 
 	/*
 	 * Breadth-first-search failed, graph got corrupted?
 	 */
 	WARN(1, "lockdep bfs error:%d\n", ret);
-
-	return 0;
 }
 
 static int noop_count(struct lock_list *entry, void *data)
@@ -1766,7 +1758,7 @@ static void print_lock_class_header(struct lock_class *class, int depth)
  */
 static void __used
 print_shortest_lock_dependencies(struct lock_list *leaf,
-				struct lock_list *root)
+				 struct lock_list *root)
 {
 	struct lock_list *entry = leaf;
 	int depth;
@@ -1788,8 +1780,6 @@ static void print_lock_class_header(struct lock_class *class, int depth)
 		entry = get_lock_parent(entry);
 		depth--;
 	} while (entry && (depth >= 0));
-
-	return;
 }
 
 static void
@@ -1848,7 +1838,7 @@ static void print_lock_class_header(struct lock_class *class, int depth)
 	printk("\n *** DEADLOCK ***\n\n");
 }
 
-static int
+static void
 print_bad_irq_dependency(struct task_struct *curr,
 			 struct lock_list *prev_root,
 			 struct lock_list *next_root,
@@ -1861,7 +1851,7 @@ static void print_lock_class_header(struct lock_class *class, int depth)
 			 const char *irqclass)
 {
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
-		return 0;
+		return;
 
 	pr_warn("\n");
 	pr_warn("=====================================================\n");
@@ -1907,19 +1897,17 @@ static void print_lock_class_header(struct lock_class *class, int depth)
 
 	pr_warn("\nthe dependencies between %s-irq-safe lock and the holding lock:\n", irqclass);
 	if (!save_trace(&prev_root->trace))
-		return 0;
+		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))
-		return 0;
+		return;
 	print_shortest_lock_dependencies(forwards_entry, next_root);
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
-
-	return 0;
 }
 
 static int
@@ -1936,23 +1924,28 @@ static void print_lock_class_header(struct lock_class *class, int depth)
 
 	this.class = hlock_class(prev);
 	ret = find_usage_backwards(&this, bit_backwards, &target_entry);
-	if (ret < 0)
-		return print_bfs_bug(ret);
+	if (ret < 0) {
+		print_bfs_bug(ret);
+		return 0;
+	}
 	if (ret == 1)
 		return ret;
 
 	that.parent = NULL;
 	that.class = hlock_class(next);
 	ret = find_usage_forwards(&that, bit_forwards, &target_entry1);
-	if (ret < 0)
-		return print_bfs_bug(ret);
+	if (ret < 0) {
+		print_bfs_bug(ret);
+		return 0;
+	}
 	if (ret == 1)
 		return ret;
 
-	return print_bad_irq_dependency(curr, &this, &that,
-			target_entry, target_entry1,
-			prev, next,
-			bit_backwards, bit_forwards, irqclass);
+	print_bad_irq_dependency(curr, &this, &that,
+				 target_entry, target_entry1,
+				 prev, next,
+				 bit_backwards, bit_forwards, irqclass);
+	return 0;
 }
 
 static const char *state_names[] = {
@@ -2055,8 +2048,7 @@ static inline void inc_chains(void)
 #endif
 
 static void
-print_deadlock_scenario(struct held_lock *nxt,
-			     struct held_lock *prv)
+print_deadlock_scenario(struct held_lock *nxt, struct held_lock *prv)
 {
 	struct lock_class *next = hlock_class(nxt);
 	struct lock_class *prev = hlock_class(prv);
@@ -2074,12 +2066,12 @@ static inline void inc_chains(void)
 	printk(" May be due to missing lock nesting notation\n\n");
 }
 
-static int
+static void
 print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 		   struct held_lock *next)
 {
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
-		return 0;
+		return;
 
 	pr_warn("\n");
 	pr_warn("============================================\n");
@@ -2098,8 +2090,6 @@ static inline void inc_chains(void)
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
-
-	return 0;
 }
 
 /*
@@ -2141,7 +2131,8 @@ static inline void inc_chains(void)
 		if (nest)
 			return 2;
 
-		return print_deadlock_bug(curr, prev, next);
+		print_deadlock_bug(curr, prev, next);
+		return 0;
 	}
 	return 1;
 }
@@ -2217,10 +2208,13 @@ static inline void inc_chains(void)
 			 */
 			save(trace);
 		}
-		return print_circular_bug(&this, target_entry, next, prev, trace);
+		print_circular_bug(&this, target_entry, next, prev, trace);
+		return 0;
+	}
+	else if (unlikely(ret < 0)) {
+		print_bfs_bug(ret);
+		return 0;
 	}
-	else if (unlikely(ret < 0))
-		return print_bfs_bug(ret);
 
 	if (!check_prev_add_irq(curr, prev, next))
 		return 0;
@@ -2261,8 +2255,10 @@ static inline void inc_chains(void)
 		debug_atomic_inc(nr_redundant);
 		return 2;
 	}
-	if (ret < 0)
-		return print_bfs_bug(ret);
+	if (ret < 0) {
+		print_bfs_bug(ret);
+		return 0;
+	}
 
 
 	if (!trace->entries && !save(trace))
@@ -2784,8 +2780,7 @@ static void check_chain_key(struct task_struct *curr)
 #endif
 }
 
-static void
-print_usage_bug_scenario(struct held_lock *lock)
+static void print_usage_bug_scenario(struct held_lock *lock)
 {
 	struct lock_class *class = hlock_class(lock);
 
@@ -2802,12 +2797,12 @@ static void check_chain_key(struct task_struct *curr)
 	printk("\n *** DEADLOCK ***\n\n");
 }
 
-static int
+static void
 print_usage_bug(struct task_struct *curr, struct held_lock *this,
 		enum lock_usage_bit prev_bit, enum lock_usage_bit new_bit)
 {
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
-		return 0;
+		return;
 
 	pr_warn("\n");
 	pr_warn("================================\n");
@@ -2837,8 +2832,6 @@ static void check_chain_key(struct task_struct *curr)
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
-
-	return 0;
 }
 
 /*
@@ -2848,8 +2841,10 @@ static void check_chain_key(struct task_struct *curr)
 valid_state(struct task_struct *curr, struct held_lock *this,
 	    enum lock_usage_bit new_bit, enum lock_usage_bit bad_bit)
 {
-	if (unlikely(hlock_class(this)->usage_mask & (1 << bad_bit)))
-		return print_usage_bug(curr, this, bad_bit, new_bit);
+	if (unlikely(hlock_class(this)->usage_mask & (1 << bad_bit))) {
+		print_usage_bug(curr, this, bad_bit, new_bit);
+		return 0;
+	}
 	return 1;
 }
 
@@ -2861,7 +2856,7 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 /*
  * print irq inversion bug:
  */
-static int
+static void
 print_irq_inversion_bug(struct task_struct *curr,
 			struct lock_list *root, struct lock_list *other,
 			struct held_lock *this, int forwards,
@@ -2872,7 +2867,7 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 	int depth;
 
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
-		return 0;
+		return;
 
 	pr_warn("\n");
 	pr_warn("========================================================\n");
@@ -2913,13 +2908,11 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 
 	pr_warn("\nthe shortest dependencies between 2nd lock and 1st lock:\n");
 	if (!save_trace(&root->trace))
-		return 0;
+		return;
 	print_shortest_lock_dependencies(other, root);
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
-
-	return 0;
 }
 
 /*
@@ -2937,13 +2930,16 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 	root.parent = NULL;
 	root.class = hlock_class(this);
 	ret = find_usage_forwards(&root, bit, &target_entry);
-	if (ret < 0)
-		return print_bfs_bug(ret);
+	if (ret < 0) {
+		print_bfs_bug(ret);
+		return 0;
+	}
 	if (ret == 1)
 		return ret;
 
-	return print_irq_inversion_bug(curr, &root, target_entry,
-					this, 1, irqclass);
+	print_irq_inversion_bug(curr, &root, target_entry,
+				this, 1, irqclass);
+	return 0;
 }
 
 /*
@@ -2961,13 +2957,16 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 	root.parent = NULL;
 	root.class = hlock_class(this);
 	ret = find_usage_backwards(&root, bit, &target_entry);
-	if (ret < 0)
-		return print_bfs_bug(ret);
+	if (ret < 0) {
+		print_bfs_bug(ret);
+		return 0;
+	}
 	if (ret == 1)
 		return ret;
 
-	return print_irq_inversion_bug(curr, &root, target_entry,
-					this, 0, irqclass);
+	print_irq_inversion_bug(curr, &root, target_entry,
+				this, 0, irqclass);
+	return 0;
 }
 
 void print_irqtrace_events(struct task_struct *curr)
@@ -3510,15 +3509,15 @@ void lockdep_init_map(struct lockdep_map *lock, const char *name,
 struct lock_class_key __lockdep_no_validate__;
 EXPORT_SYMBOL_GPL(__lockdep_no_validate__);
 
-static int
+static void
 print_lock_nested_lock_not_held(struct task_struct *curr,
 				struct held_lock *hlock,
 				unsigned long ip)
 {
 	if (!debug_locks_off())
-		return 0;
+		return;
 	if (debug_locks_silent)
-		return 0;
+		return;
 
 	pr_warn("\n");
 	pr_warn("==================================\n");
@@ -3540,8 +3539,6 @@ void lockdep_init_map(struct lockdep_map *lock, const char *name,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
-
-	return 0;
 }
 
 static int __lock_is_held(const struct lockdep_map *lock, int read);
@@ -3690,8 +3687,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	}
 	chain_key = iterate_chain_key(chain_key, class_idx);
 
-	if (nest_lock && !__lock_is_held(nest_lock, -1))
-		return print_lock_nested_lock_not_held(curr, hlock, ip);
+	if (nest_lock && !__lock_is_held(nest_lock, -1)) {
+		print_lock_nested_lock_not_held(curr, hlock, ip);
+		return 0;
+	}
 
 	if (!debug_locks_silent) {
 		WARN_ON_ONCE(depth && !hlock_class(hlock - 1)->key);
@@ -3727,14 +3726,14 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	return 1;
 }
 
-static int
-print_unlock_imbalance_bug(struct task_struct *curr, struct lockdep_map *lock,
-			   unsigned long ip)
+static void print_unlock_imbalance_bug(struct task_struct *curr,
+				       struct lockdep_map *lock,
+				       unsigned long ip)
 {
 	if (!debug_locks_off())
-		return 0;
+		return;
 	if (debug_locks_silent)
-		return 0;
+		return;
 
 	pr_warn("\n");
 	pr_warn("=====================================\n");
@@ -3752,8 +3751,6 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
-
-	return 0;
 }
 
 static int match_held_lock(const struct held_lock *hlock,
@@ -3872,8 +3869,10 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
 		return 0;
 
 	hlock = find_held_lock(curr, lock, depth, &i);
-	if (!hlock)
-		return print_unlock_imbalance_bug(curr, lock, ip);
+	if (!hlock) {
+		print_unlock_imbalance_bug(curr, lock, ip);
+		return 0;
+	}
 
 	lockdep_init_map(lock, name, key, 0);
 	class = register_lock_class(lock, subclass, 0);
@@ -3913,8 +3912,10 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
 		return 0;
 
 	hlock = find_held_lock(curr, lock, depth, &i);
-	if (!hlock)
-		return print_unlock_imbalance_bug(curr, lock, ip);
+	if (!hlock) {
+		print_unlock_imbalance_bug(curr, lock, ip);
+		return 0;
+	}
 
 	curr->lockdep_depth = i;
 	curr->curr_chain_key = hlock->prev_chain_key;
@@ -3958,16 +3959,20 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
 	 * So we're all set to release this lock.. wait what lock? We don't
 	 * own any locks, you've been drinking again?
 	 */
-	if (DEBUG_LOCKS_WARN_ON(depth <= 0))
-		 return print_unlock_imbalance_bug(curr, lock, ip);
+	if (DEBUG_LOCKS_WARN_ON(depth <= 0)) {
+		print_unlock_imbalance_bug(curr, lock, ip);
+		return 0;
+	}
 
 	/*
 	 * Check whether the lock exists in the current stack
 	 * of held locks:
 	 */
 	hlock = find_held_lock(curr, lock, depth, &i);
-	if (!hlock)
-		return print_unlock_imbalance_bug(curr, lock, ip);
+	if (!hlock) {
+		print_unlock_imbalance_bug(curr, lock, ip);
+		return 0;
+	}
 
 	if (hlock->instance == lock)
 		lock_release_holdtime(hlock);
@@ -4310,14 +4315,14 @@ void lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie cookie)
 EXPORT_SYMBOL_GPL(lock_unpin_lock);
 
 #ifdef CONFIG_LOCK_STAT
-static int
-print_lock_contention_bug(struct task_struct *curr, struct lockdep_map *lock,
-			   unsigned long ip)
+static void print_lock_contention_bug(struct task_struct *curr,
+				      struct lockdep_map *lock,
+				      unsigned long ip)
 {
 	if (!debug_locks_off())
-		return 0;
+		return;
 	if (debug_locks_silent)
-		return 0;
+		return;
 
 	pr_warn("\n");
 	pr_warn("=================================\n");
@@ -4335,8 +4340,6 @@ void lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie cookie)
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
-
-	return 0;
 }
 
 static void
-- 
1.8.3.1

[PATCH v3 02/18] locking/lockdep: Add description and explanation in lockdep design doc

[Yuyang Du]

More words are added to lockdep design document regarding key concepts,
which helps people understand the design as well as read the reports.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 Documentation/locking/lockdep-design.txt | 80 ++++++++++++++++++++++++--------
 1 file changed, 60 insertions(+), 20 deletions(-)

diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
index 49f58a0..1dcceaa 100644
--- a/Documentation/locking/lockdep-design.txt
+++ b/Documentation/locking/lockdep-design.txt
@@ -15,51 +15,91 @@ tens of thousands of) instantiations. For example a lock in the inode
 struct is one class, while each inode has its own instantiation of that
 lock class.
 
-The validator tracks the 'state' of lock-classes, and it tracks
-dependencies between different lock-classes. The validator maintains a
-rolling proof that the state and the dependencies are correct.
-
-Unlike an lock instantiation, the lock-class itself never goes away: when
-a lock-class is used for the first time after bootup it gets registered,
-and all subsequent uses of that lock-class will be attached to this
-lock-class.
+The validator tracks the 'usage state' of lock-classes, and it tracks the
+dependencies between different lock-classes. The dependency can be
+understood as lock order, where L1 -> L2 suggests L1 depends on L2, which
+can also be expressed as a forward dependency (L1 -> L2) or a backward
+dependency (L2 <- L1). From lockdep's perspective, the two locks (L1 and L2)
+are not necessarily related as opposed to in some modules an order must be
+followed. Here it just means that order ever happened. The validator
+maintains a continuing effort to prove that the lock usages and their
+dependencies are correct or the validator will shoot a splat if they are
+potentially incorrect.
+
+Unlike a lock instance, a lock-class itself never goes away: when a
+lock-class's instance is used for the first time after bootup the class gets
+registered, and all (subsequent) instances of that lock-class will be mapped
+to the lock-class.
 
 State
 -----
 
-The validator tracks lock-class usage history into 4 * nSTATEs + 1 separate
-state bits:
+The validator tracks lock-class usage history and divides the usage into
+(4 usages * n STATEs + 1) categories:
 
+Where the 4 usages can be:
 - 'ever held in STATE context'
 - 'ever held as readlock in STATE context'
 - 'ever held with STATE enabled'
 - 'ever held as readlock with STATE enabled'
 
-Where STATE can be either one of (kernel/locking/lockdep_states.h)
- - hardirq
- - softirq
+Where the n STATEs are coded in kernel/locking/lockdep_states.h and as of
+now they include:
+- hardirq
+- softirq
 
+Where the last 1 category is:
 - 'ever used'                                       [ == !unused        ]
 
-When locking rules are violated, these state bits are presented in the
-locking error messages, inside curlies. A contrived example:
+When locking rules are violated, these usage bits are presented in the
+locking error messages, inside curlies, with a total of 2 * n STATEs bits.
+See a contrived example:
 
    modprobe/2287 is trying to acquire lock:
-    (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
 
    but task is already holding lock:
-    (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
 
 
-The bit position indicates STATE, STATE-read, for each of the states listed
-above, and the character displayed in each indicates:
+For a given lock, the bit positions from left to right indicate the usage
+of the lock and readlock (if exists), for each of the n STATEs listed
+above respectively, and the character displayed at each bit position
+indicates:
 
    '.'  acquired while irqs disabled and not in irq context
    '-'  acquired in irq context
    '+'  acquired with irqs enabled
    '?'  acquired in irq context with irqs enabled.
 
-Unused mutexes cannot be part of the cause of an error.
+The bits are illustrated with an example:
+
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
+                         ||||
+                         ||| \-> softirq disabled and not in softirq context
+                         || \--> acquired in softirq context
+                         | \---> hardirq disabled and not in hardirq context
+                          \----> acquired in hardirq context
+
+
+For a given STATE, whether the lock is ever acquired in that STATE context
+and whether that STATE is enabled yields four possible cases as shown in the
+table below. It is worth noting that the bit character is able to indicate
+which exact case is for the lock as of the reporting time.
+
+   -------------------------------------------
+  |              | irq enabled | irq disabled |
+   -------------------------------------------
+  | ever in irq  |      ?      |       -      |
+   -------------------------------------------
+  | never in irq |      +      |       .      |
+   -------------------------------------------
+
+The character '-' suggests irq is disabled because if otherwise, the
+charactor '?' would have been shown instead. Similar deduction can be
+applied for '+' too.
+
+Unused locks (e.g., mutexes) cannot be part of the cause of an error.
 
 
 Single-lock state rules:
-- 
1.8.3.1

[PATCH v3 03/18] locking/lockdep: Adjust lock usage bit character checks

[Yuyang Du]

The lock usage bit characters are defined and determined with tricks. Use a
macro and add some explanation to make it a bit clearer. Then adjust the
logic to check the usage, which optimizes the code a bit.

No functional change.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c           | 21 ++++++++++++++++-----
 kernel/locking/lockdep_internals.h |  1 +
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 41137ad..db0473d 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -514,15 +514,26 @@ static inline unsigned long lock_flag(enum lock_usage_bit bit)
 
 static char get_usage_char(struct lock_class *class, enum lock_usage_bit bit)
 {
+	/*
+	 * The usage character defaults to '.' (i.e., irqs disabled and not in
+	 * irq context), which is the safest usage category.
+	 */
 	char c = '.';
 
-	if (class->usage_mask & lock_flag(bit + 2))
+	/*
+	 * The order of the following usage checks matters, which will
+	 * result in the outcome character as follows:
+	 *
+	 * - '+': irq is enabled and not in irq context
+	 * - '-': in irq context and irq is disabled
+	 * - '?': in irq context and irq is enabled
+	 */
+	if (class->usage_mask & lock_flag(bit + LOCK_USAGE_TO_ENABLED_STEP)) {
 		c = '+';
-	if (class->usage_mask & lock_flag(bit)) {
-		c = '-';
-		if (class->usage_mask & lock_flag(bit + 2))
+		if (class->usage_mask & lock_flag(bit))
 			c = '?';
-	}
+	} else if (class->usage_mask & lock_flag(bit))
+		c = '-';
 
 	return c;
 }
diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index d4c1974..2fd31d5 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -25,6 +25,7 @@ enum lock_usage_bit {
 #define LOCK_USAGE_READ_MASK 1
 #define LOCK_USAGE_DIR_MASK  2
 #define LOCK_USAGE_STATE_MASK (~(LOCK_USAGE_READ_MASK | LOCK_USAGE_DIR_MASK))
+#define LOCK_USAGE_TO_ENABLED_STEP 2
 
 /*
  * Usage-state bitmasks:
-- 
1.8.3.1

[PATCH v3 04/18] locking/lockdep: Remove useless conditional macro

[Yuyang Du]

Since #defined(CONFIG_PROVE_LOCKING) is used in the scope of #ifdef
CONFIG_PROVE_LOCKING, it can be removed.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index db0473d..eeea722 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1678,7 +1678,7 @@ unsigned long lockdep_count_backward_deps(struct lock_class *class)
 	return result;
 }
 
-#if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_PROVE_LOCKING)
+#if defined(CONFIG_TRACE_IRQFLAGS)
 /*
  * Forwards and backwards subgraph searching, for the purposes of
  * proving that two subgraphs can be connected by a new dependency
@@ -2056,7 +2056,7 @@ static inline void inc_chains(void)
 	nr_process_chains++;
 }
 
-#endif
+#endif /* CONFIG_TRACE_IRQFLAGS */
 
 static void
 print_deadlock_scenario(struct held_lock *nxt, struct held_lock *prv)
@@ -2738,7 +2738,7 @@ static inline int validate_chain(struct task_struct *curr,
 {
 	return 1;
 }
-#endif
+#endif /* CONFIG_PROVE_LOCKING */
 
 /*
  * We are building curr_chain_key incrementally, so double-check
-- 
1.8.3.1

[PATCH v3 05/18] locking/lockdep: Print the right depth for chain key colission

[Yuyang Du]

Since chains are separated by irq context, so when printing a chain the
depth should be consistent with it.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index eeea722..05c31d6 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2428,10 +2428,11 @@ static u64 print_chain_key_iteration(int class_idx, u64 chain_key)
 	struct held_lock *hlock;
 	u64 chain_key = 0;
 	int depth = curr->lockdep_depth;
-	int i;
+	int i = get_first_held_lock(curr, hlock_next);
 
-	printk("depth: %u\n", depth + 1);
-	for (i = get_first_held_lock(curr, hlock_next); i < depth; i++) {
+	printk("depth: %u (irq_context %u)\n", depth - i + 1,
+		hlock_next->irq_context);
+	for (; i < depth; i++) {
 		hlock = curr->held_locks + i;
 		chain_key = print_chain_key_iteration(hlock->class_idx, chain_key);
 
-- 
1.8.3.1

[PATCH v3 06/18] locking/lockdep: Update obsolete struct field description

[Yuyang Du]

The lock_chain struct definition has outdated comment, update it and add
struct member description.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 include/linux/lockdep.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 79c3873..37706ad 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -199,10 +199,16 @@ struct lock_list {
 };
 
 /*
- * We record lock dependency chains, so that we can cache them:
+ * struct lock_chain - we record lock dependency chains so that we can cache them
+ *
+ * @irq_context: the same as irq_context in held_lock below
+ * @depth:       the number of held locks in this chain
+ * @base:        the index in chain_hlocks for this chain
+ * @entry:       the collided lock chains in lock_chain hash list
+ * @chain_key:   the hash key of this lock_chain
  */
 struct lock_chain {
-	/* see BUILD_BUG_ON()s in lookup_chain_cache() */
+	/* see BUILD_BUG_ON()s in add_chain_cache() */
 	unsigned int			irq_context :  2,
 					depth       :  6,
 					base	    : 24;
-- 
1.8.3.1

[PATCH v3 07/18] locking/lockdep: Use lockdep_init_task for task initiation consistently

[Yuyang Du]

Despite that there is a lockdep_init_task() which does nothing, lockdep
initiates tasks by assigning lockdep fields and does so inconsistently. Fix
this by using lockdep_init_task().

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 include/linux/lockdep.h  |  7 ++++++-
 init/init_task.c         |  2 ++
 kernel/fork.c            |  3 ---
 kernel/locking/lockdep.c | 11 ++++++++---
 4 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 37706ad..267087e 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -282,6 +282,8 @@ struct held_lock {
 extern asmlinkage void lockdep_sys_exit(void);
 extern void lockdep_set_selftest_task(struct task_struct *task);
 
+extern inline void lockdep_init_task(struct task_struct *task);
+
 extern void lockdep_off(void);
 extern void lockdep_on(void);
 
@@ -406,6 +408,10 @@ static inline void lock_set_subclass(struct lockdep_map *lock,
 
 #else /* !CONFIG_LOCKDEP */
 
+static inline void lockdep_init_task(struct task_struct *task)
+{
+}
+
 static inline void lockdep_off(void)
 {
 }
@@ -498,7 +504,6 @@ enum xhlock_context_t {
 	{ .name = (_name), .key = (void *)(_key), }
 
 static inline void lockdep_invariant_state(bool force) {}
-static inline void lockdep_init_task(struct task_struct *task) {}
 static inline void lockdep_free_task(struct task_struct *task) {}
 
 #ifdef CONFIG_LOCK_STAT
diff --git a/init/init_task.c b/init/init_task.c
index 46dbf54..9460878 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -165,6 +165,8 @@ struct task_struct init_task
 	.softirqs_enabled = 1,
 #endif
 #ifdef CONFIG_LOCKDEP
+	.lockdep_depth = 0, /* no locks held yet */
+	.curr_chain_key = 0,
 	.lockdep_recursion = 0,
 #endif
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
diff --git a/kernel/fork.c b/kernel/fork.c
index 77059b2..c0d2000 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1869,9 +1869,6 @@ static __latent_entropy struct task_struct *copy_process(
 	p->pagefault_disabled = 0;
 
 #ifdef CONFIG_LOCKDEP
-	p->lockdep_depth = 0; /* no locks held yet */
-	p->curr_chain_key = 0;
-	p->lockdep_recursion = 0;
 	lockdep_init_task(p);
 #endif
 
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 05c31d6..ef59651 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -358,6 +358,13 @@ static inline u64 iterate_chain_key(u64 key, u32 idx)
 	return k0 | (u64)k1 << 32;
 }
 
+inline void lockdep_init_task(struct task_struct *task)
+{
+	task->lockdep_depth = 0; /* no locks held yet */
+	task->curr_chain_key = 0;
+	task->lockdep_recursion = 0;
+}
+
 void lockdep_off(void)
 {
 	current->lockdep_recursion++;
@@ -4496,9 +4503,7 @@ void lockdep_reset(void)
 	int i;
 
 	raw_local_irq_save(flags);
-	current->curr_chain_key = 0;
-	current->lockdep_depth = 0;
-	current->lockdep_recursion = 0;
+	lockdep_init_task(current);
 	memset(current->held_locks, 0, MAX_LOCK_DEPTH*sizeof(struct held_lock));
 	nr_hardirq_chains = 0;
 	nr_softirq_chains = 0;
-- 
1.8.3.1

[PATCH v3 08/18] locking/lockdep: Define INITIAL_CHAIN_KEY for chain keys to start with

[Yuyang Du]

Chain keys are computed using Jenkins hash function, which needs an initial
hash to start with. Dedicate a macro to make this clear and configurable. A
later patch changes this initial chain key.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 include/linux/lockdep.h  |  1 +
 init/init_task.c         |  2 +-
 kernel/locking/lockdep.c | 18 +++++++++---------
 3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 267087e..64d4565 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -224,6 +224,7 @@ struct lock_chain {
  * bitfield and hitting the BUG in hlock_class().
  */
 #define MAX_LOCKDEP_KEYS		((1UL << MAX_LOCKDEP_KEYS_BITS) - 1)
+#define INITIAL_CHAIN_KEY		0
 
 struct held_lock {
 	/*
diff --git a/init/init_task.c b/init/init_task.c
index 9460878..ff3e8bf 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -166,7 +166,7 @@ struct task_struct init_task
 #endif
 #ifdef CONFIG_LOCKDEP
 	.lockdep_depth = 0, /* no locks held yet */
-	.curr_chain_key = 0,
+	.curr_chain_key = INITIAL_CHAIN_KEY,
 	.lockdep_recursion = 0,
 #endif
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index ef59651..c6363f7 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -361,7 +361,7 @@ static inline u64 iterate_chain_key(u64 key, u32 idx)
 inline void lockdep_init_task(struct task_struct *task)
 {
 	task->lockdep_depth = 0; /* no locks held yet */
-	task->curr_chain_key = 0;
+	task->curr_chain_key = INITIAL_CHAIN_KEY;
 	task->lockdep_recursion = 0;
 }
 
@@ -867,7 +867,7 @@ static bool class_lock_list_valid(struct lock_class *c, struct list_head *h)
 static bool check_lock_chain_key(struct lock_chain *chain)
 {
 #ifdef CONFIG_PROVE_LOCKING
-	u64 chain_key = 0;
+	u64 chain_key = INITIAL_CHAIN_KEY;
 	int i;
 
 	for (i = chain->base; i < chain->base + chain->depth; i++)
@@ -2433,7 +2433,7 @@ static u64 print_chain_key_iteration(int class_idx, u64 chain_key)
 print_chain_keys_held_locks(struct task_struct *curr, struct held_lock *hlock_next)
 {
 	struct held_lock *hlock;
-	u64 chain_key = 0;
+	u64 chain_key = INITIAL_CHAIN_KEY;
 	int depth = curr->lockdep_depth;
 	int i = get_first_held_lock(curr, hlock_next);
 
@@ -2453,7 +2453,7 @@ static u64 print_chain_key_iteration(int class_idx, u64 chain_key)
 static void print_chain_keys_chain(struct lock_chain *chain)
 {
 	int i;
-	u64 chain_key = 0;
+	u64 chain_key = INITIAL_CHAIN_KEY;
 	int class_id;
 
 	printk("depth: %u\n", chain->depth);
@@ -2757,7 +2757,7 @@ static void check_chain_key(struct task_struct *curr)
 #ifdef CONFIG_DEBUG_LOCKDEP
 	struct held_lock *hlock, *prev_hlock = NULL;
 	unsigned int i;
-	u64 chain_key = 0;
+	u64 chain_key = INITIAL_CHAIN_KEY;
 
 	for (i = 0; i < curr->lockdep_depth; i++) {
 		hlock = curr->held_locks + i;
@@ -2781,7 +2781,7 @@ static void check_chain_key(struct task_struct *curr)
 
 		if (prev_hlock && (prev_hlock->irq_context !=
 							hlock->irq_context))
-			chain_key = 0;
+			chain_key = INITIAL_CHAIN_KEY;
 		chain_key = iterate_chain_key(chain_key, hlock->class_idx);
 		prev_hlock = hlock;
 	}
@@ -3694,14 +3694,14 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 		/*
 		 * How can we have a chain hash when we ain't got no keys?!
 		 */
-		if (DEBUG_LOCKS_WARN_ON(chain_key != 0))
+		if (DEBUG_LOCKS_WARN_ON(chain_key != INITIAL_CHAIN_KEY))
 			return 0;
 		chain_head = 1;
 	}
 
 	hlock->prev_chain_key = chain_key;
 	if (separate_irq_context(curr, hlock)) {
-		chain_key = 0;
+		chain_key = INITIAL_CHAIN_KEY;
 		chain_head = 1;
 	}
 	chain_key = iterate_chain_key(chain_key, class_idx);
@@ -4543,7 +4543,7 @@ static void remove_class_from_lock_chain(struct pending_free *pf,
 	return;
 
 recalc:
-	chain_key = 0;
+	chain_key = INITIAL_CHAIN_KEY;
 	for (i = chain->base; i < chain->base + chain->depth; i++)
 		chain_key = iterate_chain_key(chain_key, chain_hlocks[i] + 1);
 	if (chain->depth && chain->chain_key == chain_key)
-- 
1.8.3.1

[PATCH v3 09/18] locking/lockdep: Change the range of class_idx in held_lock struct

[Yuyang Du]

held_lock->class_idx is used to point to the class of the held lock. The
index is shifted by 1 to make index 0 mean no class, which results in class
index shifting back and forth but is not worth doing so.

The reason is: (1) there will be no "no-class" held_lock to begin with, and
(2) index 0 seems to be used for error checking, but if something wrong
indeed happended, the index can't be counted on to distinguish it as that
something won't set the class_idx to 0 on purpose to tell us it is wrong.

Therefore, change the index to start from 0. This saves a lot of
back-and-forth shifts and save a slot back to lock_classes.

Since index 0 is now used for lock class, we change the initial chain key to
-1 to avoid key collision, which is due to the fact that __jhash_mix(0, 0, 0) = 0.
Actually, the initial chain key can be any arbitrary value other than 0.

In addition, we maintain a bitmap to keep track of the used lock classes,
and we check the validity of the held lock against that bitmap.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 include/linux/lockdep.h  | 14 ++++++------
 kernel/locking/lockdep.c | 59 ++++++++++++++++++++++++++++++++----------------
 2 files changed, 46 insertions(+), 27 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 64d4565..0859d80 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -218,13 +218,8 @@ struct lock_chain {
 };
 
 #define MAX_LOCKDEP_KEYS_BITS		13
-/*
- * Subtract one because we offset hlock->class_idx by 1 in order
- * to make 0 mean no class. This avoids overflowing the class_idx
- * bitfield and hitting the BUG in hlock_class().
- */
-#define MAX_LOCKDEP_KEYS		((1UL << MAX_LOCKDEP_KEYS_BITS) - 1)
-#define INITIAL_CHAIN_KEY		0
+#define MAX_LOCKDEP_KEYS		(1UL << MAX_LOCKDEP_KEYS_BITS)
+#define INITIAL_CHAIN_KEY		-1
 
 struct held_lock {
 	/*
@@ -249,6 +244,11 @@ struct held_lock {
 	u64 				waittime_stamp;
 	u64				holdtime_stamp;
 #endif
+	/*
+	 * class_idx is zero-indexed; it points to the element in
+	 * lock_classes this held lock instance belongs to. class_idx is in
+	 * the range from 0 to (MAX_LOCKDEP_KEYS-1) inclusive.
+	 */
 	unsigned int			class_idx:MAX_LOCKDEP_KEYS_BITS;
 	/*
 	 * The lock-stack is unified in that the lock chains of interrupt
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index c6363f7..f16d2f5 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -150,17 +150,28 @@ static inline int debug_locks_off_graph_unlock(void)
 static
 #endif
 struct lock_class lock_classes[MAX_LOCKDEP_KEYS];
+static DECLARE_BITMAP(lock_classes_in_use, MAX_LOCKDEP_KEYS);
 
 static inline struct lock_class *hlock_class(struct held_lock *hlock)
 {
-	if (!hlock->class_idx) {
+	unsigned int class_idx = hlock->class_idx;
+
+	/* Don't re-read hlock->class_idx, can't use READ_ONCE() on bitfield */
+	barrier();
+
+	if (!test_bit(class_idx, lock_classes_in_use)) {
 		/*
 		 * Someone passed in garbage, we give up.
 		 */
 		DEBUG_LOCKS_WARN_ON(1);
 		return NULL;
 	}
-	return lock_classes + hlock->class_idx - 1;
+
+	/*
+	 * At this point, if the passed hlock->class_idx is still garbage,
+	 * we just have to live with it
+	 */
+	return lock_classes + class_idx;
 }
 
 #ifdef CONFIG_LOCK_STAT
@@ -604,19 +615,22 @@ static void print_lock(struct held_lock *hlock)
 	/*
 	 * We can be called locklessly through debug_show_all_locks() so be
 	 * extra careful, the hlock might have been released and cleared.
+	 *
+	 * If this indeed happens, lets pretend it does not hurt to continue
+	 * to print the lock unless the hlock class_idx does not point to a
+	 * registered class. The rationale here is: since we don't attempt
+	 * to distinguish whether we are in this situation, if it just
+	 * happened we can't count on class_idx to tell either.
 	 */
-	unsigned int class_idx = hlock->class_idx;
+	struct lock_class *lock = hlock_class(hlock);
 
-	/* Don't re-read hlock->class_idx, can't use READ_ONCE() on bitfields: */
-	barrier();
-
-	if (!class_idx || (class_idx - 1) >= MAX_LOCKDEP_KEYS) {
+	if (!lock) {
 		printk(KERN_CONT "<RELEASED>\n");
 		return;
 	}
 
 	printk(KERN_CONT "%p", hlock->instance);
-	print_lock_name(lock_classes + class_idx - 1);
+	print_lock_name(lock);
 	printk(KERN_CONT ", at: %pS\n", (void *)hlock->acquire_ip);
 }
 
@@ -871,7 +885,7 @@ static bool check_lock_chain_key(struct lock_chain *chain)
 	int i;
 
 	for (i = chain->base; i < chain->base + chain->depth; i++)
-		chain_key = iterate_chain_key(chain_key, chain_hlocks[i] + 1);
+		chain_key = iterate_chain_key(chain_key, chain_hlocks[i]);
 	/*
 	 * The 'unsigned long long' casts avoid that a compiler warning
 	 * is reported when building tools/lib/lockdep.
@@ -1146,6 +1160,7 @@ static bool is_dynamic_key(const struct lock_class_key *key)
 		return NULL;
 	}
 	nr_lock_classes++;
+	__set_bit(class - lock_classes, lock_classes_in_use);
 	debug_atomic_inc(nr_unused_locks);
 	class->key = key;
 	class->name = lock->name;
@@ -2459,7 +2474,7 @@ static void print_chain_keys_chain(struct lock_chain *chain)
 	printk("depth: %u\n", chain->depth);
 	for (i = 0; i < chain->depth; i++) {
 		class_id = chain_hlocks[chain->base + i];
-		chain_key = print_chain_key_iteration(class_id + 1, chain_key);
+		chain_key = print_chain_key_iteration(class_id, chain_key);
 
 		print_lock_name(lock_classes + class_id);
 		printk("\n");
@@ -2510,7 +2525,7 @@ static int check_no_collision(struct task_struct *curr,
 	}
 
 	for (j = 0; j < chain->depth - 1; j++, i++) {
-		id = curr->held_locks[i].class_idx - 1;
+		id = curr->held_locks[i].class_idx;
 
 		if (DEBUG_LOCKS_WARN_ON(chain_hlocks[chain->base + j] != id)) {
 			print_collision(curr, hlock, chain);
@@ -2593,7 +2608,7 @@ static inline int add_chain_cache(struct task_struct *curr,
 	if (likely(nr_chain_hlocks + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
 		chain->base = nr_chain_hlocks;
 		for (j = 0; j < chain->depth - 1; j++, i++) {
-			int lock_id = curr->held_locks[i].class_idx - 1;
+			int lock_id = curr->held_locks[i].class_idx;
 			chain_hlocks[chain->base + j] = lock_id;
 		}
 		chain_hlocks[chain->base + j] = class - lock_classes;
@@ -2773,10 +2788,12 @@ static void check_chain_key(struct task_struct *curr)
 				(unsigned long long)hlock->prev_chain_key);
 			return;
 		}
+
 		/*
-		 * Whoops ran out of static storage again?
+		 * hlock->class_idx can't go beyond MAX_LOCKDEP_KEYS, but is
+		 * it registered lock class index?
 		 */
-		if (DEBUG_LOCKS_WARN_ON(hlock->class_idx > MAX_LOCKDEP_KEYS))
+		if (DEBUG_LOCKS_WARN_ON(!test_bit(hlock->class_idx, lock_classes_in_use)))
 			return;
 
 		if (prev_hlock && (prev_hlock->irq_context !=
@@ -3622,7 +3639,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	if (DEBUG_LOCKS_WARN_ON(depth >= MAX_LOCK_DEPTH))
 		return 0;
 
-	class_idx = class - lock_classes + 1;
+	class_idx = class - lock_classes;
 
 	if (depth) {
 		hlock = curr->held_locks + depth - 1;
@@ -3684,9 +3701,9 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	 * the hash, not class->key.
 	 */
 	/*
-	 * Whoops, we did it again.. ran straight out of our static allocation.
+	 * Whoops, we did it again.. class_idx is invalid.
 	 */
-	if (DEBUG_LOCKS_WARN_ON(class_idx > MAX_LOCKDEP_KEYS))
+	if (DEBUG_LOCKS_WARN_ON(!test_bit(class_idx, lock_classes_in_use)))
 		return 0;
 
 	chain_key = curr->curr_chain_key;
@@ -3801,7 +3818,7 @@ static int match_held_lock(const struct held_lock *hlock,
 		if (DEBUG_LOCKS_WARN_ON(!hlock->nest_lock))
 			return 0;
 
-		if (hlock->class_idx == class - lock_classes + 1)
+		if (hlock->class_idx == class - lock_classes)
 			return 1;
 	}
 
@@ -3895,7 +3912,7 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
 
 	lockdep_init_map(lock, name, key, 0);
 	class = register_lock_class(lock, subclass, 0);
-	hlock->class_idx = class - lock_classes + 1;
+	hlock->class_idx = class - lock_classes;
 
 	curr->lockdep_depth = i;
 	curr->curr_chain_key = hlock->prev_chain_key;
@@ -4545,7 +4562,7 @@ static void remove_class_from_lock_chain(struct pending_free *pf,
 recalc:
 	chain_key = INITIAL_CHAIN_KEY;
 	for (i = chain->base; i < chain->base + chain->depth; i++)
-		chain_key = iterate_chain_key(chain_key, chain_hlocks[i] + 1);
+		chain_key = iterate_chain_key(chain_key, chain_hlocks[i]);
 	if (chain->depth && chain->chain_key == chain_key)
 		return;
 	/* Overwrite the chain key for concurrent RCU readers. */
@@ -4619,6 +4636,7 @@ static void zap_class(struct pending_free *pf, struct lock_class *class)
 		WRITE_ONCE(class->key, NULL);
 		WRITE_ONCE(class->name, NULL);
 		nr_lock_classes--;
+		__clear_bit(class - lock_classes, lock_classes_in_use);
 	} else {
 		WARN_ONCE(true, "%s() failed for class %s\n", __func__,
 			  class->name);
@@ -4968,6 +4986,7 @@ void __init lockdep_init(void)
 
 	printk(" memory used by lock dependency info: %zu kB\n",
 	       (sizeof(lock_classes) +
+		sizeof(lock_classes_in_use) +
 		sizeof(classhash_table) +
 		sizeof(list_entries) +
 		sizeof(list_entries_in_use) +
-- 
1.8.3.1

[PATCH v3 10/18] locking/lockdep: Remove unused argument in validate_chain() and check_deadlock()

[Yuyang Du]

The lockdep_map argument in them is not used, remove it.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
---
 kernel/locking/lockdep.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index f16d2f5..c7aec9f 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2134,8 +2134,7 @@ static inline void inc_chains(void)
  * Returns: 0 on deadlock detected, 1 on OK, 2 on recursive read
  */
 static int
-check_deadlock(struct task_struct *curr, struct held_lock *next,
-	       struct lockdep_map *next_instance, int read)
+check_deadlock(struct task_struct *curr, struct held_lock *next, int read)
 {
 	struct held_lock *prev;
 	struct held_lock *nest = NULL;
@@ -2698,8 +2697,9 @@ static inline int lookup_chain_cache_add(struct task_struct *curr,
 	return 1;
 }
 
-static int validate_chain(struct task_struct *curr, struct lockdep_map *lock,
-		struct held_lock *hlock, int chain_head, u64 chain_key)
+static int validate_chain(struct task_struct *curr,
+			  struct held_lock *hlock,
+			  int chain_head, u64 chain_key)
 {
 	/*
 	 * Trylock needs to maintain the stack of held locks, but it
@@ -2725,7 +2725,7 @@ static int validate_chain(struct task_struct *curr, struct lockdep_map *lock,
 		 * any of these scenarios could lead to a deadlock. If
 		 * All validations
 		 */
-		int ret = check_deadlock(curr, hlock, lock, hlock->read);
+		int ret = check_deadlock(curr, hlock, hlock->read);
 
 		if (!ret)
 			return 0;
@@ -2756,8 +2756,8 @@ static int validate_chain(struct task_struct *curr, struct lockdep_map *lock,
 }
 #else
 static inline int validate_chain(struct task_struct *curr,
-	       	struct lockdep_map *lock, struct held_lock *hlock,
-		int chain_head, u64 chain_key)
+				 struct held_lock *hlock,
+				 int chain_head, u64 chain_key)
 {
 	return 1;
 }
@@ -3733,7 +3733,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 		WARN_ON_ONCE(!hlock_class(hlock)->key);
 	}
 
-	if (!validate_chain(curr, lock, hlock, chain_head, chain_key))
+	if (!validate_chain(curr, hlock, chain_head, chain_key))
 		return 0;
 
 	curr->curr_chain_key = chain_key;
-- 
1.8.3.1

[PATCH v3 11/18] locking/lockdep: Update comment

[Yuyang Du]

An out-of-nowhere comment is removed. While at it, add more explanatory
comments. Such a trivial patch!

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index c7aec9f..eccfb0b 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2720,10 +2720,16 @@ static int validate_chain(struct task_struct *curr,
 		 * - is softirq-safe, if this lock is hardirq-unsafe
 		 *
 		 * And check whether the new lock's dependency graph
-		 * could lead back to the previous lock.
+		 * could lead back to the previous lock:
 		 *
-		 * any of these scenarios could lead to a deadlock. If
-		 * All validations
+		 * - within the current held-lock stack
+		 * - across our accumulated lock dependency records
+		 *
+		 * any of these scenarios could lead to a deadlock.
+		 */
+		/*
+		 * The simple case: does the current hold the same lock
+		 * already?
 		 */
 		int ret = check_deadlock(curr, hlock, hlock->read);
 
-- 
1.8.3.1

[PATCH v3 12/18] locking/lockdep: Remove unnecessary function pointer argument

[Yuyang Du]

check_prev_add() always has save_trace() as an input argument, which is
unnecessary, so remove it.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
---
 kernel/locking/lockdep.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index eccfb0b..f46695a 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2193,8 +2193,7 @@ static inline void inc_chains(void)
  */
 static int
 check_prev_add(struct task_struct *curr, struct held_lock *prev,
-	       struct held_lock *next, int distance, struct stack_trace *trace,
-	       int (*save)(struct stack_trace *trace))
+	       struct held_lock *next, int distance, struct stack_trace *trace)
 {
 	struct lock_list *uninitialized_var(target_entry);
 	struct lock_list *entry;
@@ -2234,11 +2233,11 @@ static inline void inc_chains(void)
 	if (unlikely(!ret)) {
 		if (!trace->entries) {
 			/*
-			 * If @save fails here, the printing might trigger
+			 * 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);
+			save_trace(trace);
 		}
 		print_circular_bug(&this, target_entry, next, prev, trace);
 		return 0;
@@ -2293,7 +2292,7 @@ static inline void inc_chains(void)
 	}
 
 
-	if (!trace->entries && !save(trace))
+	if (!trace->entries && !save_trace(trace))
 		return 0;
 
 	/*
@@ -2358,7 +2357,7 @@ static inline void inc_chains(void)
 		 * added:
 		 */
 		if (hlock->read != 2 && hlock->check) {
-			int ret = check_prev_add(curr, hlock, next, distance, &trace, save_trace);
+			int ret = check_prev_add(curr, hlock, next, distance, &trace);
 			if (!ret)
 				return 0;
 
-- 
1.8.3.1

[PATCH v3 13/18] locking/lockdep: Change type of the element field in circular_queue

[Yuyang Du]

The element field is an array in struct circular_queue to keep track of locks
in the search. Making it the same type as the locks avoids type cast. Also
fix a typo and elaborate the comment above struct circular_queue.

No functional change.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
---
 kernel/locking/lockdep.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index f46695a..8167d69 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1272,13 +1272,16 @@ static int add_lock_to_list(struct lock_class *this,
 #define CQ_MASK				(MAX_CIRCULAR_QUEUE_SIZE-1)
 
 /*
- * The circular_queue and helpers is used to implement the
- * breadth-first search(BFS)algorithem, by which we can build
- * the shortest path from the next lock to be acquired to the
- * previous held lock if there is a circular between them.
+ * The circular_queue and helpers are used to implement the graph
+ * breadth-first search (BFS) algorithm, by which we can determine whether
+ * there is a path from the next lock to be acquired to a previous held
+ * lock, which indicates that adding the <prev> -> <next> lock dependency
+ * produces a circle in the lock dependency graph. Breadth-first search
+ * instead of depth-first search is used for finding the shortest circular
+ * path.
  */
 struct circular_queue {
-	unsigned long element[MAX_CIRCULAR_QUEUE_SIZE];
+	struct lock_list *element[MAX_CIRCULAR_QUEUE_SIZE];
 	unsigned int  front, rear;
 };
 
@@ -1304,7 +1307,7 @@ static inline int __cq_full(struct circular_queue *cq)
 	return ((cq->rear + 1) & CQ_MASK) == cq->front;
 }
 
-static inline int __cq_enqueue(struct circular_queue *cq, unsigned long elem)
+static inline int __cq_enqueue(struct circular_queue *cq, struct lock_list *elem)
 {
 	if (__cq_full(cq))
 		return -1;
@@ -1314,7 +1317,7 @@ static inline int __cq_enqueue(struct circular_queue *cq, unsigned long elem)
 	return 0;
 }
 
-static inline int __cq_dequeue(struct circular_queue *cq, unsigned long *elem)
+static inline int __cq_dequeue(struct circular_queue *cq, struct lock_list **elem)
 {
 	if (__cq_empty(cq))
 		return -1;
@@ -1392,12 +1395,12 @@ static int __bfs(struct lock_list *source_entry,
 		goto exit;
 
 	__cq_init(cq);
-	__cq_enqueue(cq, (unsigned long)source_entry);
+	__cq_enqueue(cq, source_entry);
 
 	while (!__cq_empty(cq)) {
 		struct lock_list *lock;
 
-		__cq_dequeue(cq, (unsigned long *)&lock);
+		__cq_dequeue(cq, &lock);
 
 		if (!lock->class) {
 			ret = -2;
@@ -1421,7 +1424,7 @@ static int __bfs(struct lock_list *source_entry,
 					goto exit;
 				}
 
-				if (__cq_enqueue(cq, (unsigned long)entry)) {
+				if (__cq_enqueue(cq, entry)) {
 					ret = -1;
 					goto exit;
 				}
-- 
1.8.3.1

[PATCH v3 14/18] locking/lockdep: Change the return type of __cq_dequeue()

[Yuyang Du]

With the change, we can slightly adjust the code to iterate the queue in BFS
search, which simplifies the code. No functional change.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 8167d69..ad16793 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1317,14 +1317,21 @@ static inline int __cq_enqueue(struct circular_queue *cq, struct lock_list *elem
 	return 0;
 }
 
-static inline int __cq_dequeue(struct circular_queue *cq, struct lock_list **elem)
+/*
+ * Dequeue an element from the circular_queue, return the lock if the queue
+ * is not empty, or NULL if otherwise
+ */
+static inline struct lock_list * __cq_dequeue(struct circular_queue *cq)
 {
+	struct lock_list * lock;
+
 	if (__cq_empty(cq))
-		return -1;
+		return NULL;
 
-	*elem = cq->element[cq->front];
+	lock = cq->element[cq->front];
 	cq->front = (cq->front + 1) & CQ_MASK;
-	return 0;
+
+	return lock;
 }
 
 static inline unsigned int  __cq_get_elem_count(struct circular_queue *cq)
@@ -1376,6 +1383,7 @@ static int __bfs(struct lock_list *source_entry,
 		 int forward)
 {
 	struct lock_list *entry;
+	struct lock_list *lock;
 	struct list_head *head;
 	struct circular_queue *cq = &lock_cq;
 	int ret = 1;
@@ -1397,10 +1405,7 @@ static int __bfs(struct lock_list *source_entry,
 	__cq_init(cq);
 	__cq_enqueue(cq, source_entry);
 
-	while (!__cq_empty(cq)) {
-		struct lock_list *lock;
-
-		__cq_dequeue(cq, &lock);
+	while ((lock = __cq_dequeue(cq))) {
 
 		if (!lock->class) {
 			ret = -2;
-- 
1.8.3.1

[PATCH v3 15/18] locking/lockdep: Avoid constant checks in __bfs by using offset reference

[Yuyang Du]

In search of a dependency in the lock graph, there is contant checks for
forward or backward search. Directly reference the field offset of the
struct that differentiates the type of search to avoid those checks.

No functional change.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index ad16793..cd6792c 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1376,11 +1376,25 @@ static inline int get_lock_depth(struct lock_list *child)
 	return depth;
 }
 
+/*
+ * Return the forward or backward dependency list.
+ *
+ * @lock:   the lock_list to get its class's dependency list
+ * @offset: the offset to struct lock_class to determine whether it is
+ *          locks_after or locks_before
+ */
+static inline struct list_head *get_dep_list(struct lock_list *lock, int offset)
+{
+	void *lock_class = lock->class;
+
+	return lock_class + offset;
+}
+
 static int __bfs(struct lock_list *source_entry,
 		 void *data,
 		 int (*match)(struct lock_list *entry, void *data),
 		 struct lock_list **target_entry,
-		 int forward)
+		 int offset)
 {
 	struct lock_list *entry;
 	struct lock_list *lock;
@@ -1394,11 +1408,7 @@ static int __bfs(struct lock_list *source_entry,
 		goto exit;
 	}
 
-	if (forward)
-		head = &source_entry->class->locks_after;
-	else
-		head = &source_entry->class->locks_before;
-
+	head = get_dep_list(source_entry, offset);
 	if (list_empty(head))
 		goto exit;
 
@@ -1412,10 +1422,7 @@ static int __bfs(struct lock_list *source_entry,
 			goto exit;
 		}
 
-		if (forward)
-			head = &lock->class->locks_after;
-		else
-			head = &lock->class->locks_before;
+		head = get_dep_list(lock, offset);
 
 		DEBUG_LOCKS_WARN_ON(!irqs_disabled());
 
@@ -1448,7 +1455,8 @@ static inline int __bfs_forwards(struct lock_list *src_entry,
 			int (*match)(struct lock_list *entry, void *data),
 			struct lock_list **target_entry)
 {
-	return __bfs(src_entry, data, match, target_entry, 1);
+	return __bfs(src_entry, data, match, target_entry,
+		     offsetof(struct lock_class, locks_after));
 
 }
 
@@ -1457,7 +1465,8 @@ static inline int __bfs_backwards(struct lock_list *src_entry,
 			int (*match)(struct lock_list *entry, void *data),
 			struct lock_list **target_entry)
 {
-	return __bfs(src_entry, data, match, target_entry, 0);
+	return __bfs(src_entry, data, match, target_entry,
+		     offsetof(struct lock_class, locks_before));
 
 }
 
-- 
1.8.3.1

[PATCH v3 16/18] locking/lockdep: Combine check_noncircular and check_redundant

[Yuyang Du]

These two functions are essentially duplicates, combine them into
check_nonexistent(). Also update the comment on it.

No functional change.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 31 +++++++++++--------------------
 1 file changed, 11 insertions(+), 20 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index cd6792c..8202318 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1688,29 +1688,18 @@ unsigned long lockdep_count_backward_deps(struct lock_class *class)
 }
 
 /*
- * Prove that the dependency graph starting at <entry> can not
- * lead to <target>. Print an error and return 0 if it does.
+ * Prove that the dependency graph starting at <root> can not
+ * lead to <target>. If existent, there is a circle when adding
+ * a <target> -> <root> dependency.
+ *
+ * Print an error and return 0 if it does exist.
  */
 static noinline int
-check_noncircular(struct lock_list *root, struct lock_class *target,
-		struct lock_list **target_entry)
+check_nonexistent(struct lock_list *root, struct lock_class *target,
+		  struct lock_list **target_entry)
 {
 	int result;
 
-	debug_atomic_inc(nr_cyclic_checks);
-
-	result = __bfs_forwards(root, target, class_equal, target_entry);
-
-	return result;
-}
-
-static noinline int
-check_redundant(struct lock_list *root, struct lock_class *target,
-		struct lock_list **target_entry)
-{
-	int result;
-
-	debug_atomic_inc(nr_redundant_checks);
 
 	result = __bfs_forwards(root, target, class_equal, target_entry);
 
@@ -2246,7 +2235,8 @@ static inline void inc_chains(void)
 	 */
 	this.class = hlock_class(next);
 	this.parent = NULL;
-	ret = check_noncircular(&this, hlock_class(prev), &target_entry);
+	debug_atomic_inc(nr_cyclic_checks);
+	ret = check_nonexistent(&this, hlock_class(prev), &target_entry);
 	if (unlikely(!ret)) {
 		if (!trace->entries) {
 			/*
@@ -2298,7 +2288,8 @@ static inline void inc_chains(void)
 	 */
 	this.class = hlock_class(prev);
 	this.parent = NULL;
-	ret = check_redundant(&this, hlock_class(next), &target_entry);
+	debug_atomic_inc(nr_redundant_checks);
+	ret = check_nonexistent(&this, hlock_class(next), &target_entry);
 	if (!ret) {
 		debug_atomic_inc(nr_redundant);
 		return 2;
-- 
1.8.3.1

[PATCH v3 17/18] locking/lockdep: Update comments on dependency search

[Yuyang Du]

The breadth-first search is implemented as flat-out non-recursive now, but
the comments are still describing it as recursive, update the comments in
that regard.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 8202318..9df2b1a 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1390,6 +1390,10 @@ static inline struct list_head *get_dep_list(struct lock_list *lock, int offset)
 	return lock_class + offset;
 }
 
+/*
+ * Forward- or backward-dependency search, used for both circular dependency
+ * checking and hardirq-unsafe/softirq-unsafe checking.
+ */
 static int __bfs(struct lock_list *source_entry,
 		 void *data,
 		 int (*match)(struct lock_list *entry, void *data),
@@ -1471,12 +1475,6 @@ static inline int __bfs_backwards(struct lock_list *src_entry,
 }
 
 /*
- * Recursive, forwards-direction lock-dependency checking, used for
- * both noncyclic checking and for hardirq-unsafe/softirq-unsafe
- * checking.
- */
-
-/*
  * Print a dependency chain entry (this is only done when a deadlock
  * has been detected):
  */
@@ -2177,7 +2175,7 @@ static inline void inc_chains(void)
 
 /*
  * There was a chain-cache miss, and we are about to add a new dependency
- * to a previous lock. We recursively validate the following rules:
+ * to a previous lock. We validate the following rules:
  *
  *  - would the adding of the <prev> -> <next> dependency create a
  *    circular dependency in the graph? [== circular deadlock]
@@ -2227,11 +2225,12 @@ static inline void inc_chains(void)
 	/*
 	 * Prove that the new <prev> -> <next> dependency would not
 	 * create a circular dependency in the graph. (We do this by
-	 * forward-recursing into the graph starting at <next>, and
-	 * checking whether we can reach <prev>.)
+	 * a breadth-first search into the graph starting at <next>,
+	 * which checks whether we can reach <prev>.)
 	 *
-	 * We are using global variables to control the recursion, to
-	 * keep the stackframe size of the recursive functions low:
+	 * The search is limited by the size of the circular queue (i.e.,
+	 * MAX_CIRCULAR_QUEUE_SIZE) which keeps track of a breadth of nodes
+	 * in the graph whose neighbours are to be checked.
 	 */
 	this.class = hlock_class(next);
 	this.parent = NULL;
-- 
1.8.3.1

[PATCH v3 18/18] locking/lockdep: Add explanation to lock usage rules in lockdep design doc

[Yuyang Du]

The rules that if violated a deacklock may happen are explained in more
detail concerning both irqs and circular dependencies.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 Documentation/locking/lockdep-design.txt | 35 +++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
index 1dcceaa..83803c6 100644
--- a/Documentation/locking/lockdep-design.txt
+++ b/Documentation/locking/lockdep-design.txt
@@ -105,14 +105,24 @@ Unused locks (e.g., mutexes) cannot be part of the cause of an error.
 Single-lock state rules:
 ------------------------
 
+A lock is irq-safe means it was ever used in an irq context, while a lock
+is irq-unsafe means it was ever acquired with irq enabled.
+
 A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The
-following states are exclusive, and only one of them is allowed to be
-set for any lock-class:
+following states must be exclusive: only one of them is allowed to be set
+for any lock-class based on its usage:
+
+ <hardirq-safe> or <hardirq-unsafe>
+ <softirq-safe> or <softirq-unsafe>
 
- <hardirq-safe> and <hardirq-unsafe>
- <softirq-safe> and <softirq-unsafe>
+This is because if a lock can be used in irq (safe) then it cannot be ever
+acquired with irq enabled (unsafe). Otherwise, a deadlock may happen. For
+example, in the scenario that after this lock was acquired but before
+released, if the context is interrupted this lock will be attempted to
+acquire twice, which creates a deadlock, sometimes referred to as lock
+recursion deadlock.
 
-The validator detects and reports lock usage that violate these
+The validator detects and reports lock usage that violates these
 single-lock state rules.
 
 Multi-lock dependency rules:
@@ -121,15 +131,20 @@ Multi-lock dependency rules:
 The same lock-class must not be acquired twice, because this could lead
 to lock recursion deadlocks.
 
-Furthermore, two locks may not be taken in different order:
+Furthermore, two locks can not be taken in inverse order:
 
  <L1> -> <L2>
  <L2> -> <L1>
 
-because this could lead to lock inversion deadlocks. (The validator
-finds such dependencies in arbitrary complexity, i.e. there can be any
-other locking sequence between the acquire-lock operations, the
-validator will still track all dependencies between locks.)
+because it could lead to a deadlock - sometimes referred to as lock
+inversion deadlock - as attempts to acquire the two locks form a circle
+which could lead to two contexts waiting for each other permanently, namely
+the two contexts are holding one lock while waiting for acquiring the other
+in an inverse order. The validator will find such circle in arbitrary
+complexity. In other words, there can be any number of locking sequences
+between two acquire-lock operations (holding one lock while acquiring
+another); the validator will still find whether these locks can be acquired
+in a circular fashion.
 
 Furthermore, the following usage based lock dependencies are not allowed
 between any two lock-classes:
-- 
1.8.3.1

Question on a lockdep test case about mixed read-write ABBA

[Yuyang Du]

Hi Peter,

I observed this test case you wrote in Commit: e9149858974606
("locking/lockdep/selftests: Add mixed read-write ABBA").

static void rwsem_ABBA2(void)
{
       RSL(X1);
       ML(Y1);
       MU(Y1);
       RSU(X1);

       ML(Y1);
       RSL(X1);
       RSU(X1);
       MU(Y1); // should fail
}

Why should it fail? This is not a deadlock, right? The depencencies
would be built by lockdep though; that results in a false positive.

Thanks,
Yuyang