lockdep: Avoid to modify chain keys in validate_chain()

Chris Wilson reported a problem spotted by check_chain_key(): a chain
key got changed in validate_chain() because we modify the ->read in
validate_chain() to skip checks for dependency adding, and ->read is
taken into calculation for chain key since commit f611e8cf98
("lockdep: Take read/write status in consideration when generate
chainkey").

Fix this by avoiding to modify ->read in validate_chain() based on two
facts: a) since we now support recursive read lock detection, there is
no need to skip checks for dependency adding for recursive readers, b)
since we have a), there is only one case left (nest_lock) where we want
to skip checks in validate_chain(), we simply remove the modification
for ->read and rely on the return value of check_deadlock() to skip the
dependency adding.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201102053743.450459-1-boqun.feng@gmail.com

kernel/locking/lockdep.c

@@ -2765,7 +2765,9 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
  * (Note that this has to be done separately, because the graph cannot
  * detect such classes of deadlocks.)
  *
- * Returns: 0 on deadlock detected, 1 on OK, 2 on recursive read
+ * Returns: 0 on deadlock detected, 1 on OK, 2 if another lock with the same
+ * lock class is held but nest_lock is also held, i.e. we rely on the
+ * nest_lock to avoid the deadlock.
  */
 static int
 check_deadlock(struct task_struct *curr, struct held_lock *next)
@@ -2788,7 +2790,7 @@ check_deadlock(struct task_struct *curr, struct held_lock *next)
 		 * lock class (i.e. read_lock(lock)+read_lock(lock)):
 		 */
 		if ((next->read == 2) && prev->read)
-			return 2;
+			continue;
 
 		/*
 		 * We're holding the nest_lock, which serializes this lock's
@@ -3592,16 +3594,13 @@ static int validate_chain(struct task_struct *curr,
 
 		if (!ret)
 			return 0;
-		/*
-		 * Mark recursive read, as we jump over it when
-		 * building dependencies (just like we jump over
-		 * trylock entries):
-		 */
-		if (ret == 2)
-			hlock->read = 2;
 		/*
 		 * Add dependency only if this lock is not the head
-		 * of the chain, and if it's not a secondary read-lock:
+		 * of the chain, and if the new lock introduces no more
+		 * lock dependency (because we already hold a lock with the
+		 * same lock class) nor deadlock (because the nest_lock
+		 * serializes nesting locks), see the comments for
+		 * check_deadlock().
 		 */
 		if (!chain_head && ret != 2) {
 			if (!check_prevs_add(curr, hlock))