coredump: zap_threads: comments && use while_each_thread()

No changes in fs/exec.o

The for_each_process() loop in zap_threads() is very subtle, it is not
clear why we don't race with fork/exit/exec.  Add the fat comment.

Also, change the code to use while_each_thread().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -1517,7 +1517,7 @@ static void zap_process(struct task_struct *start)
 			sigaddset(&t->pending.signal, SIGKILL);
 			signal_wake_up(t, 1);
 		}
-	} while ((t = next_thread(t)) != start);
+	} while_each_thread(start, t);
 }
 
 static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
@@ -1539,7 +1539,36 @@ static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 
 	if (atomic_read(&mm->mm_users) == mm->core_waiters + 1)
 		goto done;
-
+	/*
+	 * We should find and kill all tasks which use this mm, and we should
+	 * count them correctly into mm->core_waiters. We don't take tasklist
+	 * lock, but this is safe wrt:
+	 *
+	 * fork:
+	 *	None of sub-threads can fork after zap_process(leader). All
+	 *	processes which were created before this point should be
+	 *	visible to zap_threads() because copy_process() adds the new
+	 *	process to the tail of init_task.tasks list, and lock/unlock
+	 *	of ->siglock provides a memory barrier.
+	 *
+	 * do_exit:
+	 *	The caller holds mm->mmap_sem. This means that the task which
+	 *	uses this mm can't pass exit_mm(), so it can't exit or clear
+	 *	its ->mm.
+	 *
+	 * de_thread:
+	 *	It does list_replace_rcu(&leader->tasks, &current->tasks),
+	 *	we must see either old or new leader, this does not matter.
+	 *	However, it can change p->sighand, so lock_task_sighand(p)
+	 *	must be used. Since p->mm != NULL and we hold ->mmap_sem
+	 *	it can't fail.
+	 *
+	 *	Note also that "g" can be the old leader with ->mm == NULL
+	 *	and already unhashed and thus removed from ->thread_group.
+	 *	This is OK, __unhash_process()->list_del_rcu() does not
+	 *	clear the ->next pointer, we will find the new leader via
+	 *	next_thread().
+	 */
 	rcu_read_lock();
 	for_each_process(g) {
 		if (g == tsk->group_leader)
@@ -1549,17 +1578,13 @@ static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 		do {
 			if (p->mm) {
 				if (p->mm == mm) {
-					/*
-					 * p->sighand can't disappear, but
-					 * may be changed by de_thread()
-					 */
 					lock_task_sighand(p, &flags);
 					zap_process(p);
 					unlock_task_sighand(p, &flags);
 				}
 				break;
 			}
-		} while ((p = next_thread(p)) != g);
+		} while_each_thread(g, p);
 	}
 	rcu_read_unlock();
 done: