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 434010980bdf9b3ae6b66f2982da1b5a92e20c47..bd2bea963364c757e90db1e47ce557a94d053c0b 100644 (file)
--- a/lib/is_single_threaded.c
+++ b/lib/is_single_threaded.c
@@ -22,8 +22,6 @@ bool current_is_single_threaded(void)
        struct task_struct *p, *t;
        bool ret;
 
-       might_sleep();
-
        if (atomic_read(&task->signal->live) != 1)
                return false;
 
@@ -31,7 +29,6 @@ bool current_is_single_threaded(void)
                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 @@ bool current_is_single_threaded(void)
                                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;
 }