debug_show_all_locks() tries to grab tasklist_lock for two seconds, but
calling while_each_thread() without tasklist_lock held is not safe.
See commit 4449a51a7c281602 ("vm_is_stack: use for_each_thread() rather
then buggy while_each_thread()") for more information.

Change debug_show_all_locks() from "do_each_thread()/while_each_thread()
with possibility of missing tasklist_lock" to "for_each_process_thread()
with RCU", and add a call to touch_all_softlockup_watchdogs() like
show_state_filter() does.

Signed-off-by: Tetsuo Handa <penguin-ker...@i-love.sakura.ne.jp>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Ingo Molnar <mi...@redhat.com>
---
 kernel/locking/lockdep.c | 43 ++++++++-----------------------------------
 1 file changed, 8 insertions(+), 35 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 0233863..94f4d21 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4451,8 +4451,6 @@ void debug_check_no_locks_held(void)
 void debug_show_all_locks(void)
 {
        struct task_struct *g, *p;
-       int count = 10;
-       int unlock = 1;
 
        if (unlikely(!debug_locks)) {
                pr_warn("INFO: lockdep is turned off.\n");
@@ -4460,30 +4458,8 @@ void debug_show_all_locks(void)
        }
        pr_warn("\nShowing all locks held in the system:\n");
 
-       /*
-        * Here we try to get the tasklist_lock as hard as possible,
-        * if not successful after 2 seconds we ignore it (but keep
-        * trying). This is to enable a debug printout even if a
-        * tasklist_lock-holding task deadlocks or crashes.
-        */
-retry:
-       if (!read_trylock(&tasklist_lock)) {
-               if (count == 10)
-                       pr_warn("hm, tasklist_lock locked, retrying... ");
-               if (count) {
-                       count--;
-                       pr_cont(" #%d", 10-count);
-                       mdelay(200);
-                       goto retry;
-               }
-               pr_cont(" ignoring it.\n");
-               unlock = 0;
-       } else {
-               if (count != 10)
-                       pr_cont(" locked it.\n");
-       }
-
-       do_each_thread(g, p) {
+       rcu_read_lock();
+       for_each_process_thread(g, p) {
                /*
                 * It's not reliable to print a task's held locks
                 * if it's not sleeping (or if it's not the current
@@ -4491,19 +4467,16 @@ void debug_show_all_locks(void)
                 */
                if (p->state == TASK_RUNNING && p != current)
                        continue;
-               if (p->lockdep_depth)
-                       lockdep_print_held_locks(p);
-               if (!unlock)
-                       if (read_trylock(&tasklist_lock))
-                               unlock = 1;
+               if (!p->lockdep_depth)
+                       continue;
+               lockdep_print_held_locks(p);
                touch_nmi_watchdog();
-       } while_each_thread(g, p);
+               touch_all_softlockup_watchdogs();
+       }
+       rcu_read_unlock();
 
        pr_warn("\n");
        pr_warn("=============================================\n\n");
-
-       if (unlock)
-               read_unlock(&tasklist_lock);
 }
 EXPORT_SYMBOL_GPL(debug_show_all_locks);
 #endif
-- 
1.8.3.1

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