[PATCH v3 (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.

debug_show_all_lock_holders() skips current thread if the caller is
holding no lock, for reporting RCU lock taken inside that function is
generally useless.

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
I couldn't catch Peter's question at
https://lkml.kernel.org/r/Y+oY3Xd43nNnkDSB@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx .
I consider that this patch as-is is helpful, for not all TASK_RUNNING threads
are actually running on some CPU, aren't they? If we show backtrace of only
TASK_RUNNING threads which are running on some CPU, we fail to get hints for
TASK_RUNNING threads which are not running on some CPU. Therefore, I consider
that showing backtrace of TASK_RUNNING threads which are not running on some
CPU is better than not showing.

Changes in v3:
  Unshare debug_show_all_lock_holders() and debug_show_all_locks(),
  suggested by Ingo Molnar <mingo@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 |  5 +++++
 kernel/hung_task.c          |  2 +-
 kernel/locking/lockdep.c    | 28 ++++++++++++++++++++++++++++
 3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/include/linux/debug_locks.h b/include/linux/debug_locks.h
index dbb409d77d4f..0567d5ce5b4a 100644
--- a/include/linux/debug_locks.h
+++ b/include/linux/debug_locks.h
@@ -50,6 +50,7 @@ extern int debug_locks_off(void);
 #ifdef CONFIG_LOCKDEP
 extern void debug_show_all_locks(void);
 extern void debug_show_held_locks(struct task_struct *task);
+extern void debug_show_all_lock_holders(void);
 extern void debug_check_no_locks_freed(const void *from, unsigned long len);
 extern void debug_check_no_locks_held(void);
 #else
@@ -61,6 +62,10 @@ 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 322813366c6c..12aa473b11bd 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -215,7 +215,7 @@ 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 50d4863974e7..208292813776 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -32,6 +32,7 @@
 #include <linux/sched/clock.h>
 #include <linux/sched/task.h>
 #include <linux/sched/mm.h>
+#include <linux/sched/debug.h>
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/proc_fs.h>
@@ -6512,6 +6513,33 @@ void debug_show_all_locks(void)
 	pr_warn("=============================================\n\n");
 }
 EXPORT_SYMBOL_GPL(debug_show_all_locks);
+
+void debug_show_all_lock_holders(void)
+{
+	struct task_struct *g, *p;
+
+	if (unlikely(!debug_locks)) {
+		pr_warn("INFO: lockdep is turned off.\n");
+		return;
+	}
+	pr_warn("\nShowing all threads with 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;
+		sched_show_task(p);
+		lockdep_print_held_locks(p);
+		touch_nmi_watchdog();
+		touch_all_softlockup_watchdogs();
+	}
+	rcu_read_unlock();
+
+	pr_warn("\n");
+	pr_warn("=============================================\n\n");
+}
 #endif
 
 /*
-- 
2.34.1