Date:   Thu, 11 Aug 2022 20:32:28 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Dooks <ben.dooks@...ive.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Xiaoming Ni <nixiaoming@...wei.com>,
        John Ogness <john.ogness@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: [PATCH] locking/lockdep: add debug_show_all_lock_holders()

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.

Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
I wish that lockdep continues tracking locks even after something went
wrong (i.e. debug_locks remains 1), for recently I sometimes encounter
problems that disable lockdep during boot stage.

It would be noisy to report possibility of e.g. circular locking dependency
every time due to keeping debug_locks enabled. But tracking locks even
after something went wrong will help debug_show_all_lock_holders() to
survive problems during boot stage.

Changing debug_locks behavior is a future patch. For now, this patch alone
will help debugging Greg's usb.git#usb-testing tree which is generating
many "INFO: task hung in" reports.

 include/linux/debug_locks.h |  5 +++++
 kernel/hung_task.c          |  2 +-
 kernel/locking/lockdep.c    | 32 ++++++++++++++++++++++++++++++++
 3 files changed, 38 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 bb2354f73ded..18e22bbb714f 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -205,7 +205,7 @@ 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 64a13eb56078..d06254108eb7 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>
 
@@ -6509,6 +6510,37 @@ 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;
+		/*
+		 * Assuming that the caller of this function is in a process
+		 * context without any locks held, skip current thread which is
+		 * holding only RCU read lock.
+		 */
+		if (p == current)
+			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
 
 /*
-- 
2.18.4

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