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From:   Boqun Feng <boqun.feng@gmail.com>
To:     linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: [RFC v7 09/19] lockdep: Support deadlock detection for recursive read locks in check_noncircular()
Date:   Fri,  7 Aug 2020 15:42:28 +0800
Message-Id: <20200807074238.1632519-10-boqun.feng@gmail.com>
In-Reply-To: <20200807074238.1632519-1-boqun.feng@gmail.com>
References: <20200807074238.1632519-1-boqun.feng@gmail.com>
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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@gmail.com>
---
 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