[PATCH v2 (repost)] locking/lockdep: add debug_show_all_lock_holders()

[Tetsuo Handa]

Currently, check_hung_uninterruptible_tasks() reports details of locks
held in the system. Also, lockdep_print_held_locks() does not report
details of locks held by a thread if that thread is in TASK_RUNNING state.
Several years of experience of debugging without vmcore tells me that
these limitations have been a barrier for understanding what went wrong
in syzbot's "INFO: task hung in" reports.

I initially thought that the cause of "INFO: task hung in" reports is
due to over-stressing. But I understood that over-stressing is unlikely.
I now consider that there likely is a deadlock/livelock bug where lockdep
cannot report as a deadlock when "INFO: task hung in" is reported.

A typical case is that thread-1 is waiting for something to happen (e.g.
wait_event_*()) with a lock held. When thread-2 tries to hold that lock
using e.g. mutex_lock(), check_hung_uninterruptible_tasks() reports that
thread-2 is hung and thread-1 is holding a lock which thread-2 is trying
to hold. But currently check_hung_uninterruptible_tasks() cannot report
the exact location of thread-1 which gives us an important hint for
understanding why thread-1 is holding that lock for so long period.

When check_hung_uninterruptible_tasks() reports a thread waiting for a
lock, it is important to report backtrace of threads which already held
that lock. Therefore, allow check_hung_uninterruptible_tasks() to report
the exact location of threads which is holding any lock.

To deduplicate code, share debug_show_all_{locks,lock_holders}() using
a flag. As a side effect of sharing, __debug_show_all_locks() skips
current thread if the caller is holding no lock, for reporting RCU lock
taken inside __debug_show_all_locks() is generally useless.

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
---
Changes in v2:
  Share debug_show_all_lock_holders() and debug_show_all_locks(),
  suggested by Waiman Long <longman@xxxxxxxxxx>.

 include/linux/debug_locks.h | 17 ++++++++++++++++-
 kernel/hung_task.c          |  2 +-
 kernel/locking/lockdep.c    | 14 +++++++++++---
 3 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/include/linux/debug_locks.h b/include/linux/debug_locks.h
index dbb409d77d4f..b45c89fadfe4 100644
--- a/include/linux/debug_locks.h
+++ b/include/linux/debug_locks.h
@@ -48,7 +48,18 @@ extern int debug_locks_off(void);
 #endif
 
 #ifdef CONFIG_LOCKDEP
-extern void debug_show_all_locks(void);
+extern void __debug_show_all_locks(bool show_stack);
+
+static inline void debug_show_all_locks(void)
+{
+	__debug_show_all_locks(false);
+}
+
+static inline void debug_show_all_lock_holders(void)
+{
+	__debug_show_all_locks(true);
+}
+
 extern void debug_show_held_locks(struct task_struct *task);
 extern void debug_check_no_locks_freed(const void *from, unsigned long len);
 extern void debug_check_no_locks_held(void);
@@ -61,6 +72,10 @@ static inline void debug_show_held_locks(struct task_struct *task)
 {
 }
 
+static inline void debug_show_all_lock_holders(void)
+{
+}
+
 static inline void
 debug_check_no_locks_freed(const void *from, unsigned long len)
 {
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index c71889f3f3fc..5fba784258b7 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -213,7 +213,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
  unlock:
 	rcu_read_unlock();
 	if (hung_task_show_lock)
-		debug_show_all_locks();
+		debug_show_all_lock_holders();
 
 	if (hung_task_show_all_bt) {
 		hung_task_show_all_bt = false;
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index e3375bc40dad..b3da133825cc 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -55,6 +55,7 @@
 #include <linux/rcupdate.h>
 #include <linux/kprobes.h>
 #include <linux/lockdep.h>
+#include <linux/sched/debug.h>
 
 #include <asm/sections.h>
 
@@ -6487,7 +6488,7 @@ void debug_check_no_locks_held(void)
 EXPORT_SYMBOL_GPL(debug_check_no_locks_held);
 
 #ifdef __KERNEL__
-void debug_show_all_locks(void)
+void __debug_show_all_locks(bool show_stack)
 {
 	struct task_struct *g, *p;
 
@@ -6495,12 +6496,19 @@ void debug_show_all_locks(void)
 		pr_warn("INFO: lockdep is turned off.\n");
 		return;
 	}
-	pr_warn("\nShowing all locks held in the system:\n");
+	if (show_stack)
+		pr_warn("\nShowing all threads with locks held in the system:\n");
+	else
+		pr_warn("\nShowing all locks held in the system:\n");
 
 	rcu_read_lock();
 	for_each_process_thread(g, p) {
 		if (!p->lockdep_depth)
 			continue;
+		if (p == current && p->lockdep_depth == 1)
+			continue;
+		if (show_stack)
+			sched_show_task(p);
 		lockdep_print_held_locks(p);
 		touch_nmi_watchdog();
 		touch_all_softlockup_watchdogs();
@@ -6510,7 +6518,7 @@ void debug_show_all_locks(void)
 	pr_warn("\n");
 	pr_warn("=============================================\n\n");
 }
-EXPORT_SYMBOL_GPL(debug_show_all_locks);
+EXPORT_SYMBOL_GPL(__debug_show_all_locks);
 #endif
 
 /*
-- 
2.18.4