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Message-ID: <c8893402-d1b2-9fad-3aad-d130c5f5923b@I-love.SAKURA.ne.jp>
Date: Thu, 29 Dec 2022 09:25:33 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
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>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 (repost)] locking/lockdep: add
debug_show_all_lock_holders()
Hello, Andrew.
Since neither Peter nor Ingo is responding, would you take this patch via
your tree?
On 2022/11/21 19:10, 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@...ove.SAKURA.ne.jp>
> Acked-by: Waiman Long <longman@...hat.com>
> ---
> Changes in v2:
> Share debug_show_all_lock_holders() and debug_show_all_locks(),
> suggested by Waiman Long <longman@...hat.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
>
> /*
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