vm_is_stack: use for_each_thread() rather then buggy while_each_thread()

Aleksei hit the soft lockup during reading /proc/PID/smaps.  David
investigated the problem and suggested the right fix.

while_each_thread() is racy and should die, this patch updates
vm_is_stack().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Aleksei Besogonov <alex.besogonov@gmail.com>
Tested-by: Aleksei Besogonov <alex.besogonov@gmail.com>
Suggested-by: David Rientjes <rientjes@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 6 deletions

diff --git a/mm/util.c b/mm/util.c
index 7b6608df2ee8..093c973f1697 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -183,17 +183,14 @@ pid_t vm_is_stack(struct task_struct *task,
 
         if (in_group) {
                 struct task_struct *t;
-                rcu_read_lock();
-                if (!pid_alive(task))
-                        goto done;
 
-                t = task;
-                do {
+                rcu_read_lock();
+                for_each_thread(task, t) {
                         if (vm_is_stack_for_task(t, vma)) {
                                 ret = t->pid;
                                 goto done;
                         }
-                } while_each_thread(task, t);
+                }
 done:
                 rcu_read_unlock();
         }