Messages in this thread Patch in this message | | | Date | Mon, 21 Nov 2022 19:10:58 +0900 | From | Tetsuo Handa <> | Subject | [PATCH v2 (repost)] locking/lockdep: add debug_show_all_lock_holders() |
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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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