locking/lockdep: Use for_each_process_thread() for debug_show_all_locks()

debug_show_all_locks() tries to grab the tasklist_lock for two seconds, but
calling while_each_thread() without tasklist_lock held is not safe.

See the following commit for more information:

  4449a51a7c281602 ("vm_is_stack: use for_each_thread() rather then buggy while_each_thread()")

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-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1523011279-8206-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 8 insertions, 35 deletions

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 023386338269..94f4d21ff66d 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4451,8 +4451,6 @@ EXPORT_SYMBOL_GPL(debug_check_no_locks_held);
 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 @@ retry:
                  */
                 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