ptrace children revamp

ptrace no longer fiddles with the children/sibling links, and the
old ptrace_children list is gone.  Now ptrace, whether of one's own
children or another's via PTRACE_ATTACH, just uses the new ptraced
list instead.

There should be no user-visible difference that matters.  The only
change is the order in which do_wait() sees multiple stopped
children and stopped ptrace attachees.  Since wait_task_stopped()
was changed earlier so it no longer reorders the children list, we
already know this won't cause any new problems.

Signed-off-by: Roland McGrath <roland@redhat.com>

1 files changed, 24 insertions, 13 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index e337390fce01..8392a9da6450 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -33,13 +33,9 @@
  */
 void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
 {
-        BUG_ON(!list_empty(&child->ptrace_list));
-        if (child->parent == new_parent)
-                return;
-        list_add(&child->ptrace_list, &child->parent->ptrace_children);
-        remove_parent(child);
+        BUG_ON(!list_empty(&child->ptrace_entry));
+        list_add(&child->ptrace_entry, &new_parent->ptraced);
         child->parent = new_parent;
-        add_parent(child);
 }
  
 /*
@@ -73,12 +69,8 @@ void __ptrace_unlink(struct task_struct *child)
         BUG_ON(!child->ptrace);
 
         child->ptrace = 0;
-        if (ptrace_reparented(child)) {
-                list_del_init(&child->ptrace_list);
-                remove_parent(child);
-                child->parent = child->real_parent;
-                add_parent(child);
-        }
+        child->parent = child->real_parent;
+        list_del_init(&child->ptrace_entry);
 
         if (task_is_traced(child))
                 ptrace_untrace(child);
@@ -492,15 +484,34 @@ int ptrace_traceme(void)
         /*
          * Are we already being traced?
          */
+repeat:
         task_lock(current);
         if (!(current->ptrace & PT_PTRACED)) {
+                /*
+                 * See ptrace_attach() comments about the locking here.
+                 */
+                unsigned long flags;
+                if (!write_trylock_irqsave(&tasklist_lock, flags)) {
+                        task_unlock(current);
+                        do {
+                                cpu_relax();
+                        } while (!write_can_lock(&tasklist_lock));
+                        goto repeat;
+                }
+
                 ret = security_ptrace(current->parent, current,
                                       PTRACE_MODE_ATTACH);
+
                 /*
                  * Set the ptrace bit in the process ptrace flags.
+                 * Then link us on our parent's ptraced list.
                  */
-                if (!ret)
+                if (!ret) {
                         current->ptrace |= PT_PTRACED;
+                        __ptrace_link(current, current->real_parent);
+                }
+
+                write_unlock_irqrestore(&tasklist_lock, flags);
         }
         task_unlock(current);
         return ret;