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: David S. Miller <davem@davemloft.net>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

4 files changed, 93 insertions, 31 deletions

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 2486eb4edbf1..1bfdc30bb0af 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -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:
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index d38a64362973..6999e64fc248 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -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);
 
diff --git a/kernel/lockdep_internals.h b/kernel/lockdep_internals.h
index c3600a091a28..68d44ec77ab5 100644
--- a/kernel/lockdep_internals.h
+++ b/kernel/lockdep_internals.h
@@ -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:
diff --git a/kernel/lockdep_proc.c b/kernel/lockdep_proc.c
index 9b0e940e2545..6252ff799d19 100644
--- a/kernel/lockdep_proc.c
+++ b/kernel/lockdep_proc.c
@@ -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);