[RFC v7 09/19] lockdep: Support deadlock detection for recursive read locks in check_noncircular()

From: Boqun Feng
Date: Fri Aug 07 2020 - 03:44:44 EST


Currently, lockdep only has limit support for deadlock detection for
recursive read locks.

This patch support deadlock detection for recursive read locks. The
basic idea is:

We are about to add dependency B -> A in to the dependency graph, we use
check_noncircular() to find whether we have a strong dependency path
A -> .. -> B so that we have a strong dependency circle (a closed strong
dependency path):

A -> .. -> B -> A

, which doesn't have two adjacent dependencies as -(*R)-> L -(S*)->.

Since A -> .. -> B is already a strong dependency path, so if either
B -> A is -(E*)-> or A -> .. -> B is -(*N)->, the circle A -> .. -> B ->
A is strong, otherwise not. So we introduce a new match function
hlock_conflict() to replace the class_equal() for the deadlock check in
check_noncircular().

Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
kernel/locking/lockdep.c | 43 ++++++++++++++++++++++++++++++++--------
1 file changed, 35 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 62f7f88e3673..e5b2c1cf4286 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1838,10 +1838,37 @@ static inline bool class_equal(struct lock_list *entry, void *data)
return entry->class == data;
}

+/*
+ * We are about to add B -> A into the dependency graph, and in __bfs() a
+ * strong dependency path A -> .. -> B is found: hlock_class equals
+ * entry->class.
+ *
+ * We will have a deadlock case (conflict) if A -> .. -> B -> A is a strong
+ * dependency cycle, that means:
+ *
+ * Either
+ *
+ * a) B -> A is -(E*)->
+ *
+ * or
+ *
+ * b) A -> .. -> B is -(*N)-> (i.e. A -> .. -(*N)-> B)
+ *
+ * as then we don't have -(*R)-> -(S*)-> in the cycle.
+ */
+static inline bool hlock_conflict(struct lock_list *entry, void *data)
+{
+ struct held_lock *hlock = (struct held_lock *)data;
+
+ return hlock_class(hlock) == entry->class && /* Found A -> .. -> B */
+ (hlock->read == 0 || /* B -> A is -(E*)-> */
+ !entry->only_xr); /* A -> .. -> B is -(*N)-> */
+}
+
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 lock_list *target,
+ struct held_lock *check_src,
+ struct held_lock *check_tgt)
{
struct task_struct *curr = current;
struct lock_list *parent;
@@ -1950,13 +1977,13 @@ unsigned long lockdep_count_backward_deps(struct lock_class *class)
* <target> or not.
*/
static noinline enum bfs_result
-check_path(struct lock_class *target, struct lock_list *src_entry,
+check_path(struct held_lock *target, struct lock_list *src_entry,
+ bool (*match)(struct lock_list *entry, void *data),
struct lock_list **target_entry)
{
enum bfs_result ret;

- ret = __bfs_forwards(src_entry, (void *)target, class_equal,
- target_entry);
+ ret = __bfs_forwards(src_entry, target, match, target_entry);

if (unlikely(bfs_error(ret)))
print_bfs_bug(ret);
@@ -1983,7 +2010,7 @@ check_noncircular(struct held_lock *src, struct held_lock *target,

debug_atomic_inc(nr_cyclic_checks);

- ret = check_path(hlock_class(target), &src_entry, &target_entry);
+ ret = check_path(target, &src_entry, hlock_conflict, &target_entry);

if (unlikely(ret == BFS_RMATCH)) {
if (!*trace) {
@@ -2021,7 +2048,7 @@ check_redundant(struct held_lock *src, struct held_lock *target)

debug_atomic_inc(nr_redundant_checks);

- ret = check_path(hlock_class(target), &src_entry, &target_entry);
+ ret = check_path(target, &src_entry, class_equal, &target_entry);

if (ret == BFS_RMATCH)
debug_atomic_inc(nr_redundant);
--
2.28.0