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Subject[PATCH v2 (repost)] 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.

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@I-love.SAKURA.ne.jp>
Acked-by: Waiman Long <longman@redhat.com>
---
Changes in v2:
Share debug_show_all_lock_holders() and debug_show_all_locks(),
suggested by Waiman Long <longman@redhat.com>.

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
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