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.
debug_show_all_lock_holders() skips current thread if the caller is
holding no lock, for reporting RCU lock taken inside that function is
generally useless.
Signed-off-by: Tetsuo Handa <[email protected]>
---
I couldn't catch Peter's question at
https://lkml.kernel.org/r/[email protected] .
I consider that this patch as-is is helpful, for not all TASK_RUNNING threads
are actually running on some CPU, aren't they? If we show backtrace of only
TASK_RUNNING threads which are running on some CPU, we fail to get hints for
TASK_RUNNING threads which are not running on some CPU. Therefore, I consider
that showing backtrace of TASK_RUNNING threads which are not running on some
CPU is better than not showing.
Changes in v3:
Unshare debug_show_all_lock_holders() and debug_show_all_locks(),
suggested by Ingo Molnar <[email protected]>.
Changes in v2:
Share debug_show_all_lock_holders() and debug_show_all_locks(),
suggested by Waiman Long <[email protected]>.
include/linux/debug_locks.h | 5 +++++
kernel/hung_task.c | 2 +-
kernel/locking/lockdep.c | 28 ++++++++++++++++++++++++++++
3 files changed, 34 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 322813366c6c..12aa473b11bd 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -215,7 +215,7 @@ 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 50d4863974e7..208292813776 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -32,6 +32,7 @@
#include <linux/sched/clock.h>
#include <linux/sched/task.h>
#include <linux/sched/mm.h>
+#include <linux/sched/debug.h>
#include <linux/delay.h>
#include <linux/module.h>
#include <linux/proc_fs.h>
@@ -6512,6 +6513,33 @@ 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;
+ if (p == current && p->lockdep_depth == 1)
+ 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
/*
--
2.34.1
Ingo or Peter, can we make progress on this patch?
Linus wants this patch be handled by locking people, if this patch is acceptable.
https://lkml.kernel.org/r/CAHk-=wjbu9USn=hVWQ9v9t1H+8R6qXj8REkm36==w10zM0cM6g@mail.gmail.com
On 2023/04/04 18:01, Hillf Danton wrote:
>> 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.
>
> Yeah more info should have been printed for udevd/5109 to help work out
> the cause of task hung [1].
>
> [1] https://lore.kernel.org/lkml/[email protected]/
>>
>> debug_show_all_lock_holders() skips current thread if the caller is
>> holding no lock, for reporting RCU lock taken inside that function is
>> generally useless.
>>
>> Signed-off-by: Tetsuo Handa <[email protected]>
>> ---
>
> Acked-by: Hillf Danton <[email protected]>
>
>> I couldn't catch Peter's question at
>> https://lkml.kernel.org/r/[email protected] .
>> I consider that this patch as-is is helpful, for not all TASK_RUNNING threads
>> are actually running on some CPU, aren't they? If we show backtrace of only
>> TASK_RUNNING threads which are running on some CPU, we fail to get hints for
>> TASK_RUNNING threads which are not running on some CPU. Therefore, I consider
>> that showing backtrace of TASK_RUNNING threads which are not running on some
>> CPU is better than not showing.