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Subject[PATCH] lockdep: Print proper scenario if cross deadlock detected at acquisition time
Date
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@linux.vnet.ibm.com>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
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
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