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Commit 419ca3f1 authored by David Miller's avatar David Miller Committed by Ingo Molnar
Browse files

lockdep: fix combinatorial explosion in lock subgraph traversal 



When we traverse the graph, either forwards or backwards, we
are interested in whether a certain property exists somewhere
in a node reachable in the graph.

Therefore it is never necessary to traverse through a node more
than once to get a correct answer to the given query.

Take advantage of this property using a global ID counter so that we
need not clear all the markers in all the lock_class entries before
doing a traversal.  A new ID is choosen when we start to traverse, and
we continue through a lock_class only if it's ID hasn't been marked
with the new value yet.

This short-circuiting is essential especially for high CPU count
systems.  The scheduler has a runqueue per cpu, and needs to take
two runqueue locks at a time, which leads to long chains of
backwards and forwards subgraphs from these runqueue lock nodes.
Without the short-circuit implemented here, a graph traversal on
a runqueue lock can take up to (1 << (N - 1)) checks on a system
with N cpus.

For anything more than 16 cpus or so, lockdep will eventually bring
the machine to a complete standstill.

Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 6e86841d

include/linux/lockdep.h

+1 −0
Original line number Diff line number Diff line
@@ -89,6 +89,7 @@ struct lock_class { 


	struct lockdep_subclass_key	*key;

	unsigned int			subclass;

	unsigned int			dep_gen_id;



	/*

	 * IRQ/softirq usage tracking bits:

kernel/lockdep.c

+86 −0
Original line number Diff line number Diff line
@@ -372,6 +372,19 @@ unsigned int nr_process_chains; 
unsigned int max_lockdep_depth;

unsigned int max_recursion_depth;



static unsigned int lockdep_dependency_gen_id;



static bool lockdep_dependency_visit(struct lock_class *source,

				     unsigned int depth)

{

	if (!depth)

		lockdep_dependency_gen_id++;

	if (source->dep_gen_id == lockdep_dependency_gen_id)

		return true;

	source->dep_gen_id = lockdep_dependency_gen_id;

	return false;

}



#ifdef CONFIG_DEBUG_LOCKDEP

/*

 * We cannot printk in early bootup code. Not even early_printk()
@@ -558,6 +571,9 @@ static void print_lock_dependencies(struct lock_class *class, int depth) 
{

	struct lock_list *entry;



	if (lockdep_dependency_visit(class, depth))

		return;



	if (DEBUG_LOCKS_WARN_ON(depth >= 20))

		return;
@@ -959,6 +975,67 @@ static int noinline print_infinite_recursion_bug(void) 
	return 0;

}



unsigned long __lockdep_count_forward_deps(struct lock_class *class,

					   unsigned int depth)

{

	struct lock_list *entry;

	unsigned long ret = 1;



	if (lockdep_dependency_visit(class, depth))

		return 0;



	/*

	 * Recurse this class's dependency list:

	 */

	list_for_each_entry(entry, &class->locks_after, entry)

		ret += __lockdep_count_forward_deps(entry->class, depth + 1);



	return ret;

}



unsigned long lockdep_count_forward_deps(struct lock_class *class)

{

	unsigned long ret, flags;



	local_irq_save(flags);

	__raw_spin_lock(&lockdep_lock);

	ret = __lockdep_count_forward_deps(class, 0);

	__raw_spin_unlock(&lockdep_lock);

	local_irq_restore(flags);



	return ret;

}



unsigned long __lockdep_count_backward_deps(struct lock_class *class,

					    unsigned int depth)

{

	struct lock_list *entry;

	unsigned long ret = 1;



	if (lockdep_dependency_visit(class, depth))

		return 0;

	/*

	 * Recurse this class's dependency list:

	 */

	list_for_each_entry(entry, &class->locks_before, entry)

		ret += __lockdep_count_backward_deps(entry->class, depth + 1);



	return ret;

}



unsigned long lockdep_count_backward_deps(struct lock_class *class)

{

	unsigned long ret, flags;



	local_irq_save(flags);

	__raw_spin_lock(&lockdep_lock);

	ret = __lockdep_count_backward_deps(class, 0);

	__raw_spin_unlock(&lockdep_lock);

	local_irq_restore(flags);



	return ret;

}



/*

 * Prove that the dependency graph starting at <entry> can not

 * lead to <target>. Print an error and return 0 if it does.
@@ -968,6 +1045,9 @@ check_noncircular(struct lock_class *source, unsigned int depth) 
{

	struct lock_list *entry;



	if (lockdep_dependency_visit(source, depth))

		return 1;



	debug_atomic_inc(&nr_cyclic_check_recursions);

	if (depth > max_recursion_depth)

		max_recursion_depth = depth;
@@ -1011,6 +1091,9 @@ find_usage_forwards(struct lock_class *source, unsigned int depth) 
	struct lock_list *entry;

	int ret;



	if (lockdep_dependency_visit(source, depth))

		return 1;



	if (depth > max_recursion_depth)

		max_recursion_depth = depth;

	if (depth >= RECURSION_LIMIT)
@@ -1050,6 +1133,9 @@ find_usage_backwards(struct lock_class *source, unsigned int depth) 
	struct lock_list *entry;

	int ret;



	if (lockdep_dependency_visit(source, depth))

		return 1;



	if (!__raw_spin_is_locked(&lockdep_lock))

		return DEBUG_LOCKS_WARN_ON(1);

kernel/lockdep_internals.h

+3 −0
Original line number Diff line number Diff line
@@ -53,6 +53,9 @@ extern unsigned int nr_process_chains; 
extern unsigned int max_lockdep_depth;

extern unsigned int max_recursion_depth;



extern unsigned long lockdep_count_forward_deps(struct lock_class *);

extern unsigned long lockdep_count_backward_deps(struct lock_class *);



#ifdef CONFIG_DEBUG_LOCKDEP

/*

 * Various lockdep statistics:

kernel/lockdep_proc.c

+3 −31
Original line number Diff line number Diff line
@@ -63,34 +63,6 @@ static void l_stop(struct seq_file *m, void *v) 
{

}



static unsigned long count_forward_deps(struct lock_class *class)

{

	struct lock_list *entry;

	unsigned long ret = 1;



	/*

	 * Recurse this class's dependency list:

	 */

	list_for_each_entry(entry, &class->locks_after, entry)

		ret += count_forward_deps(entry->class);



	return ret;

}



static unsigned long count_backward_deps(struct lock_class *class)

{

	struct lock_list *entry;

	unsigned long ret = 1;



	/*

	 * Recurse this class's dependency list:

	 */

	list_for_each_entry(entry, &class->locks_before, entry)

		ret += count_backward_deps(entry->class);



	return ret;

}



static void print_name(struct seq_file *m, struct lock_class *class)

{

	char str[128];
@@ -124,10 +96,10 @@ static int l_show(struct seq_file *m, void *v) 
#ifdef CONFIG_DEBUG_LOCKDEP

	seq_printf(m, " OPS:%8ld", class->ops);

#endif

	nr_forward_deps = count_forward_deps(class);

	nr_forward_deps = lockdep_count_forward_deps(class);

	seq_printf(m, " FD:%5ld", nr_forward_deps);



	nr_backward_deps = count_backward_deps(class);

	nr_backward_deps = lockdep_count_backward_deps(class);

	seq_printf(m, " BD:%5ld", nr_backward_deps);



	get_usage_chars(class, &c1, &c2, &c3, &c4);
@@ -350,7 +322,7 @@ static int lockdep_stats_show(struct seq_file *m, void *v) 
		if (class->usage_mask & LOCKF_ENABLED_HARDIRQS_READ)

			nr_hardirq_read_unsafe++;



		sum_forward_deps += count_forward_deps(class);

		sum_forward_deps += lockdep_count_forward_deps(class);

	}

#ifdef CONFIG_DEBUG_LOCKDEP

	DEBUG_LOCKS_WARN_ON(debug_atomic_read(&nr_unused_locks) != nr_unused);