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Subject[PATCH 1/7] lockdep: Print a nice description of an irq locking issue
From: Steven Rostedt <srostedt@redhat.com>

Locking order inversion due to interrupts is a subtle problem.
When a locking inversion due to interrupts is discovered by lockdep,
it currently reports something like this:

[ INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected ]

And then writes the locks that are involved as well as back traces.
But several developers are confused by what a HARDIRQ->safe to unsafe
issue is all about, and sometimes even blow it off as a bug in lockdep.
As it is not obvious when lockdep describes this about a lock that
is never taken in interrupt context.

After explaining the problems that lockdep is reporting, I decided
to add a description of the problem in visual form. Now the following
is shown:

---
other info that might help us debug this:
Possible interrupt unsafe locking scenario:

CPU0 CPU1
---- ----
lock(lockA);
local_irq_disable();
lock(&rq->lock);
lock(lockA);
<Interrupt>
lock(&rq->lock);
*** DEADLOCK ***

---
The above is the case when the unsafe lock is taken while holding
a lock taken in irq context. But when a lock is taken that also
grabs a unsafe lock, the call chain is shown:

---
other info that might help us debug this:
Chain exists of:
&rq->lock --> lockA --> lockC

Possible interrupt unsafe locking scenario:

CPU0 CPU1
---- ----
lock(lockC);
local_irq_disable();
lock(&rq->lock);
lock(lockA);
<Interrupt>
lock(&rq->lock);
*** DEADLOCK ***

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/lockdep.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 70 insertions(+), 0 deletions(-)
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 0d2058d..bb77c030 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -490,6 +490,18 @@ void get_usage_chars(struct lock_class *class, char usage[LOCK_USAGE_CHARS])
usage[i] = '\0';
}

+static int __print_lock_name(struct lock_class *class)
+{
+ char str[KSYM_NAME_LEN];
+ const char *name;
+
+ name = class->name;
+ if (!name)
+ name = __get_key_name(class->key, str);
+
+ return printk("%s", name);
+}
+
static void print_lock_name(struct lock_class *class)
{
char str[KSYM_NAME_LEN], usage[LOCK_USAGE_CHARS];
@@ -1325,6 +1337,62 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
return;
}

+static void
+print_irq_lock_scenario(struct lock_list *safe_entry,
+ struct lock_list *unsafe_entry,
+ struct held_lock *prev,
+ struct held_lock *next)
+{
+ struct lock_class *safe_class = safe_entry->class;
+ struct lock_class *unsafe_class = unsafe_entry->class;
+ struct lock_class *middle_class = hlock_class(prev);
+
+ if (middle_class == safe_class)
+ middle_class = hlock_class(next);
+
+ /*
+ * A direct locking problem where unsafe_class lock is taken
+ * directly by safe_class lock, then all we need to show
+ * is the deadlock scenario, as it is obvious that the
+ * unsafe lock is taken under the safe lock.
+ *
+ * But if there is a chain instead, where the safe lock takes
+ * an intermediate lock (middle_class) where this lock is
+ * not the same as the safe lock, then the lock chain is
+ * used to describe the problem. Otherwise we would need
+ * to show a different CPU case for each link in the chain
+ * from the safe_class lock to the unsafe_class lock.
+ */
+ if (middle_class != unsafe_class) {
+ printk("Chain exists of:\n ");
+ __print_lock_name(safe_class);
+ printk(" --> ");
+ __print_lock_name(middle_class);
+ printk(" --> ");
+ __print_lock_name(unsafe_class);
+ printk("\n\n");
+ }
+
+ printk(" Possible interrupt unsafe locking scenario:\n\n");
+ printk(" CPU0 CPU1\n");
+ printk(" ---- ----\n");
+ printk(" lock(");
+ __print_lock_name(unsafe_class);
+ printk(");\n");
+ printk(" local_irq_disable();\n");
+ printk(" lock(");
+ __print_lock_name(safe_class);
+ printk(");\n");
+ printk(" lock(");
+ __print_lock_name(middle_class);
+ printk(");\n");
+ printk(" <Interrupt>\n");
+ printk(" lock(");
+ __print_lock_name(safe_class);
+ printk(");\n");
+ printk("\n *** DEADLOCK ***\n\n");
+}
+
static int
print_bad_irq_dependency(struct task_struct *curr,
struct lock_list *prev_root,
@@ -1376,6 +1444,8 @@ print_bad_irq_dependency(struct task_struct *curr,
print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);

printk("\nother info that might help us debug this:\n\n");
+ print_irq_lock_scenario(backwards_entry, forwards_entry, prev, next);
+
lockdep_print_held_locks(curr);

printk("\nthe dependencies between %s-irq-safe lock", irqclass);
--
1.7.2.3



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