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>
diff --git a/kernel/lockdep_proc.c b/kernel/lockdep_proc.c
index 9b0e940..6252ff7 100644
--- a/kernel/lockdep_proc.c
+++ b/kernel/lockdep_proc.c
@@ -63,34 +63,6 @@
{
}
-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 @@
#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 @@
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);