[RFC tip/locking/lockdep v5 05/17] lockdep: Extend __bfs() to work with multiple kinds of dependencies

Boqun Feng
2018-02-22T07:06:13Z

Now we have four kinds of dependencies in the dependency graph, and not
all the pathes carry strong dependencies, for example:

Given lock A, B, C, if we have:

CPU1 CPU2
============= ==============
write_lock(A); read_lock(B);
read_lock(B); write_lock(C);

then we have dependencies A--(NR)-->B, and B--(RN)-->C, (NR and
RN are to indicate the dependency kind), A actually doesn't have
strong dependency to C(IOW, C doesn't depend on A), to see this,
let's say we have a third CPU3 doing:

CPU3:
=============
write_lock(C);
write_lock(A);

, this is not a deadlock. However if we change the read_lock()
on CPU2 to a write_lock(), it's a deadlock then.

So A --(NR)--> B --(RN)--> C is not a strong dependency path but
A --(NR)--> B --(NN)-->C is a strong dependency path.

We can generalize this as: If a path of dependencies doesn't have two
adjacent dependencies as (*R)--L-->(R*), where L is some lock, it is a
strong dependency path, otherwise it's not.

Now our mission is to make __bfs() traverse only the strong dependency
paths, which is simple: we record whether we have -(*R)-> at the current
tail of the path in lock_list::is_rr, and whenever we pick a dependency
in the traverse, we 1) make sure we don't pick a -(R*)-> dependency if
our current tail is -(*R)-> and 2) greedily pick a -(*N)-> as hard as
possible.

With this extension for __bfs(), we now need to initialize the root of
__bfs() properly(with a correct ->is_rr), to do so, we introduce some
helper functions, which also cleans up a little bit for the __bfs() root
initialization code.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
include/linux/lockdep.h | 2 +
kernel/locking/lockdep.c | 116 ++++++++++++++++++++++++++++++++++++++++-------
2 files changed, 101 insertions(+), 17 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index ab1e5a7d8864..a1f91f8680bd 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -189,6 +189,8 @@ struct lock_list {
int distance;
/* bitmap of different dependencies from head to this */
u16 dep;
+ /* used by BFS to record whether this is picked as a recursive read */
+ u16 is_rr;

/*
* The parent field is used to implement breadth-first search, and the
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index acd25bfc336d..07bcfaac6fe2 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1040,6 +1040,89 @@ static inline unsigned int calc_dep(int prev, int next)
return 1U << __calc_dep_bit(prev, next);
}

+/*
+ * return -1 if no proper dependency could be picked
+ * return 0 if a -(*N)-> dependency could be picked
+ * return 1 if only a -(*R)-> dependency could be picked
+ *
+ * N: non-recursive lock
+ * R: recursive read lock
+ */
+static inline int pick_dep(u16 is_rr, u16 cap_dep)
+{
+ if (is_rr) { /* could only pick -(N*)-> */
+ if (cap_dep & DEP_NN_MASK)
+ return 0;
+ else if (cap_dep & DEP_NR_MASK)
+ return 1;
+ else
+ return -1;
+ } else {
+ if (cap_dep & DEP_NN_MASK || cap_dep & DEP_RN_MASK)
+ return 0;
+ else
+ return 1;
+ }
+}
+
+/*
+ * Initialize a lock_list entry @lock belonging to @class as the root for a BFS
+ * search.
+ */
+static inline void __bfs_init_root(struct lock_list *lock,
+ struct lock_class *class)
+{
+ lock->class = class;
+ lock->parent = NULL;
+ lock->is_rr = 0;
+}
+
+/*
+ * Initialize a lock_list entry @lock based on a lock acquisition @hlock as the
+ * root for a BFS search.
+ */
+static inline void bfs_init_root(struct lock_list *lock,
+ struct held_lock *hlock)
+{
+ __bfs_init_root(lock, hlock_class(hlock));
+ lock->is_rr = (hlock->read == 2);
+}
+
+/*
+ * Breadth-First Search to find a strong path in the dependency graph.
+ *
+ * @source_entry: the source of the path we are searching for.
+ * @data: data used for the second parameter of @match function
+ * @match: match function for the search
+ * @target_entry: pointer to the target of a matched path
+ * @forward: direction of path, the lockdep dependency forward or backward
+ *
+ * We may have multiple edges(considering different kinds of dependencies, e.g.
+ * -(NR)-> and -(RN)->) between two nodes in the dependency graph, which may
+ * undermine the normal BFS algorithm, however, we are lucky because: in the
+ * search, for each pair of adjacent nodes, we can pick the edge greedily:
+ *
+ * Say we have nodes L0, L1 and L2, and we already pick edge from L0 to
+ * L1, and we are going to pick the edge from L1 to L2, because we are
+ * picking edges to *strong* path, that means if we pick -(*R)-> for L0 to
+ * L1 (i.e. we pick L0 -(*R)-> L1), we can not pick any L1 -(R*)-> L2.
+ *
+ * And if we pick L0 -(NR)-> L1, and we have edge 1) L1-(NR)->L2 and 2)
+ * L1-(NN)->L2, we can greedily pick edge 2) for the path searching,
+ * because a) if ...->L0-(NR)->L1-(NR)->L2->... could cause a deadlock,
+ * then so does ...->L0->(NR)->L1-(NN)->L2->... and b) picking 2) means we
+ * can pick any kinds of edge for L2 to the next node, so we can search
+ * more deadlock cases then picking 1).
+ *
+ * So we have two rules of picking edges in BFS:
+ *
+ * "strong": if the previous edge we pick is -(*R)->, we must pick -(N*)->,
+ * otherwise, we can pick any kind.
+ * "greedy": if we can pick -(*R)-> or -(*N)-> (if both of them satisfies the
+ * "strong" rule), we always pick -(*N)-> ones.
+ *
+ * And that's how pick_dep() is implemeneted.
+ */
static enum bfs_result __bfs(struct lock_list *source_entry,
void *data,
int (*match)(struct lock_list *entry, void *data),
@@ -1050,6 +1133,7 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
struct list_head *head;
struct circular_queue *cq = &lock_cq;
enum bfs_result ret = BFS_RNOMATCH;
+ int is_rr, next_is_rr;

if (match(source_entry, data)) {
*target_entry = source_entry;
@@ -1095,11 +1179,18 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
else
head = &lock->class->locks_before;

+ is_rr = lock->is_rr;
+
DEBUG_LOCKS_WARN_ON(!irqs_disabled());

list_for_each_entry_rcu(entry, head, entry) {
unsigned int cq_depth;

+ next_is_rr = pick_dep(is_rr, entry->dep);
+ if (next_is_rr < 0)
+ continue;
+ entry->is_rr = next_is_rr;
+
visit_lock_entry(entry, lock);
if (match(entry, data)) {
*target_entry = entry;
@@ -1326,8 +1417,7 @@ unsigned long lockdep_count_forward_deps(struct lock_class *class)
unsigned long ret, flags;
struct lock_list this;

- this.parent = NULL;
- this.class = class;
+ __bfs_init_root(&this, class);

local_irq_save(flags);
arch_spin_lock(&lockdep_lock);
@@ -1353,8 +1443,7 @@ unsigned long lockdep_count_backward_deps(struct lock_class *class)
unsigned long ret, flags;
struct lock_list this;

- this.parent = NULL;
- this.class = class;
+ __bfs_init_root(&this, class);

local_irq_save(flags);
arch_spin_lock(&lockdep_lock);
@@ -1641,17 +1730,14 @@ check_usage(struct task_struct *curr, struct held_lock *prev,
struct lock_list *uninitialized_var(target_entry);
struct lock_list *uninitialized_var(target_entry1);

- this.parent = NULL;
-
- this.class = hlock_class(prev);
+ bfs_init_root(&this, prev);
ret = find_usage_backwards(&this, bit_backwards, &target_entry);
if (bfs_error(ret))
return print_bfs_bug(ret);
if (ret == BFS_RNOMATCH)
return 1;

- that.parent = NULL;
- that.class = hlock_class(next);
+ bfs_init_root(&that, next);
ret = find_usage_forwards(&that, bit_forwards, &target_entry1);
if (bfs_error(ret))
return print_bfs_bug(ret);
@@ -1907,8 +1993,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
* We are using global variables to control the recursion, to
* keep the stackframe size of the recursive functions low:
*/
- this.class = hlock_class(next);
- this.parent = NULL;
+ bfs_init_root(&this, next);
ret = check_noncircular(&this, hlock_class(prev), &target_entry);
if (unlikely(ret == BFS_RMATCH)) {
if (!trace->entries) {
@@ -1966,8 +2051,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
/*
* Is the <prev> -> <next> link redundant?
*/
- this.class = hlock_class(prev);
- this.parent = NULL;
+ bfs_init_root(&this, prev);
ret = check_redundant(&this, hlock_class(next), &target_entry);
if (ret == BFS_RMATCH) {
debug_atomic_inc(nr_redundant);
@@ -2710,8 +2794,7 @@ check_usage_forwards(struct task_struct *curr, struct held_lock *this,
struct lock_list root;
struct lock_list *uninitialized_var(target_entry);

- root.parent = NULL;
- root.class = hlock_class(this);
+ bfs_init_root(&root, this);
ret = find_usage_forwards(&root, bit, &target_entry);
if (bfs_error(ret))
return print_bfs_bug(ret);
@@ -2734,8 +2817,7 @@ check_usage_backwards(struct task_struct *curr, struct held_lock *this,
struct lock_list root;
struct lock_list *uninitialized_var(target_entry);

- root.parent = NULL;
- root.class = hlock_class(this);
+ bfs_init_root(&root, this);
ret = find_usage_backwards(&root, bit, &target_entry);
if (bfs_error(ret))
return print_bfs_bug(ret);
--
2.16.1