[PATCH] lockdep: Print proper scenario if cross deadlock detected at acquisition time

[Boqun Feng]

For a potential deadlock about CROSSRELEASE as follow:

	P1		P2
	===========	=============
	lock(A)
	lock(X)
			lock(A)
			commit(X)

	A: normal lock, X: cross lock

, we could detect it at two places:

1. commit time:

	We have run P1 first, and have dependency A --> X in graph, and
	then we run P2, and find the deadlock.

2. acquisition time:

	We have run P2 first, and have dependency X --> A, in
	graph(because another P3 may run previously and is acquiring for
	lock X), and then we run P1 and find the deadlock.

In current print_circular_lock_scenario(), for 1) we could print the
right scenario and note that's a deadlock related to CROSSRELEASE,
however for 2) we print the scenario as a normal lockdep deadlock,
instead we print something like:

| [   35.310179] ======================================================
| [   35.310749] WARNING: possible circular locking dependency detected
| [   35.310749] 4.13.0-rc4+ #1 Not tainted
| [   35.310749] ------------------------------------------------------
| [   35.310749] torture_onoff/766 is trying to acquire lock:
| [   35.313943]  ((complete)&st->done){+.+.}, at: [<ffffffffb905f5a6>] takedown_cpu+0x86/0xf0
| [   35.313943] 
| [   35.313943] but task is already holding lock:
| [   35.313943]  (sparse_irq_lock){+.+.}, at: [<ffffffffb90c5e42>] irq_lock_sparse+0x12/0x20
| [   35.313943] 
| [   35.313943] which lock already depends on the new lock.
...
| [   35.313943] other info that might help us debug this:
| [   35.313943] 
| [   35.313943]  Possible unsafe locking scenario:
| [   35.313943] 
| [   35.313943]        CPU0                    CPU1
| [   35.313943]        ----                    ----
| [   35.313943]   lock(sparse_irq_lock);
| [   35.313943]                                lock((complete)&st->done);
| [   35.313943]                                lock(sparse_irq_lock);
| [   35.313943]   lock((complete)&st->done);
| [   35.313943] 
| [   35.313943]  *** DEADLOCK ***

It's better to print a proper scenario related to CROSSRELEASE to help
users find their bugs more easily, so improve this.

Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Byungchul Park <byungchul.park@xxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
The sample of print_circular_lock_scenario() is from Paul Mckenney.

 kernel/locking/lockdep.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 44c8d0d17170..67a407bcc814 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1156,6 +1156,23 @@ print_circular_lock_scenario(struct held_lock *src,
 		__print_lock_name(target);
 		printk(KERN_CONT ");\n");
 		printk("\n *** DEADLOCK ***\n\n");
+	} else if (cross_lock(src->instance)) {
+		printk(" Possible unsafe locking scenario by crosslock:\n\n");
+		printk("       CPU0                    CPU1\n");
+		printk("       ----                    ----\n");
+		printk("  lock(");
+		__print_lock_name(target);
+		printk(KERN_CONT ");\n");
+		printk("  lock(");
+		__print_lock_name(source);
+		printk(KERN_CONT ");\n");
+		printk("                               lock(");
+		__print_lock_name(parent == source ? target : parent);
+		printk(KERN_CONT ");\n");
+		printk("                               unlock(");
+		__print_lock_name(source);
+		printk(KERN_CONT ");\n");
+		printk("\n *** DEADLOCK ***\n\n");
 	} else {
 		printk(" Possible unsafe locking scenario:\n\n");
 		printk("       CPU0                    CPU1\n");
-- 
2.14.1