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

[Waiman Long]

On 9/16/22 11:57, Tetsuo Handa wrote:
> 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>
> ---
> 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 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..7870f7e5c46b 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>
>   
> @@ -6485,7 +6486,7 @@ 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;
>   
> @@ -6493,12 +6494,19 @@ 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();
> @@ -6508,7 +6516,7 @@ 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
>   
>   /*
Acked-by: Waiman Long <longman@xxxxxxxxxx>