kernel: is_current_single_threaded: don't use ->mmap_sem
is_current_single_threaded() can safely miss a freshly forked CLONE_VM
task, but in this case it must not miss its parent. That is why we take
mm->mmap_sem for writing to make sure a thread/task with the same ->mm
can't pass exit_mm() and disappear.
However we can avoid ->mmap_sem and rely on rcu/barriers:
- if we do not see the exiting parent on thread/process list
we see the result of list_del_rcu(), in this case we must
also see the result of list_add_rcu() which does wmb().
- if we do see the parent but its ->mm == NULL, we need rmb()
to make sure we can't miss the child.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/lib/is_single_threaded.c b/lib/is_single_threaded.c
index 4340109..bd2bea9 100644
--- a/lib/is_single_threaded.c
+++ b/lib/is_single_threaded.c
@@ -22,8 +22,6 @@
struct task_struct *p, *t;
bool ret;
- might_sleep();
-
if (atomic_read(&task->signal->live) != 1)
return false;
@@ -31,7 +29,6 @@
return true;
ret = false;
- down_write(&mm->mmap_sem);
rcu_read_lock();
for_each_process(p) {
if (unlikely(p->flags & PF_KTHREAD))
@@ -45,12 +42,17 @@
goto found;
if (likely(t->mm))
break;
+ /*
+ * t->mm == NULL. Make sure next_thread/next_task
+ * will see other CLONE_VM tasks which might be
+ * forked before exiting.
+ */
+ smp_rmb();
} while_each_thread(p, t);
}
ret = true;
found:
rcu_read_unlock();
- up_write(&mm->mmap_sem);
return ret;
}