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>

1 files changed, 6 insertions, 4 deletions

diff --git a/lib/is_single_threaded.c b/lib/is_single_threaded.c
index 434010980bdf..bd2bea963364 100644
--- 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;
 }