Commit 967cc537 authored Jul 09, 2009 by Oleg Nesterov Committed by James Morris Jul 17, 2009

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>

parent 5bb459bb

lib/is_single_threaded.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -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;
		}