tree d99845169cde5e238a02d1e984ff9aa65b66482f
parent 5bb459bb45d1ad3c177485dcf0af01580aa31125
author Oleg Nesterov <oleg@redhat.com> 1247174929 +0200
committer James Morris <jmorris@namei.org> 1247785891 +1000

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>