Hello.

Can we add lockdep functions which print trace of threads with locks held?

For example,

void debug_show_map_users(struct lockdep_map *map)
{
        struct task_struct *g, *p;
        struct held_lock *hlock;
        int i, depth;

        rcu_read_lock();
        for_each_process_thread(g, p) {
                depth = p->lockdep_depth;
                hlock = p->held_locks;
                for (i = 0; i < depth; i++)
                        if (map == hlock[i]->instance) {
                                touch_nmi_watchdog();
                                touch_all_softlockup_watchdogs();
                                sched_show_task(p);
                                lockdep_print_held_locks(p);
                                break;
                        }
        }
        rcu_read_unlock();
}

is for replacing debug_show_all_locks() in oom_reap_task() because
we are interested in only threads holding specific mm->mmap_sem.

For example

void debug_show_relevant_tasks(struct task_struct *origin)
{
        struct task_struct *g, *p;
        struct held_lock *i_hlock, *j_hlock;
        int i, j, i_depth, j_depth;

        rcu_read_lock();
        i_depth = origin->lockdep_depth;
        i_hlock = origin->held_locks;
        for_each_process_thread(g, p) {
                j_depth = p->lockdep_depth;
                j_hlock = p->held_locks;
                for (i = 0; i < i_depth; i++)
                        for (j = 0; j < j_depth; j++)
                                if (i_hlock[i]->instance == 
j_hlock[j]->instance)
                                        goto hit;
                continue;
hit:
                touch_nmi_watchdog();
                touch_all_softlockup_watchdogs();
                sched_show_task(p);
                lockdep_print_held_locks(p);
        }
        rcu_read_unlock();
}

or

void debug_show_all_locked_tasks(void)
{
        struct task_struct *g, *p;

        rcu_read_lock();
        for_each_process_thread(g, p) {
                if (p->lockdep_depth == 0)
                        continue;
                touch_nmi_watchdog();
                touch_all_softlockup_watchdogs();
                sched_show_task(p);
                lockdep_print_held_locks(p);
        }
        rcu_read_unlock();
}

are for replacing debug_show_all_locks() in check_hung_task() for cases like
https://syzkaller.appspot.com/bug?id=26aa22915f5e3b7ca2cfca76a939f12c25d624db
because we are interested in only threads holding locks.

SysRq-t is too much but SysRq-w is useless for killable/interruptible threads...

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