android_kernel_google_msm/lib/is_single_threaded.c
Oleg Nesterov 967cc53711 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>
2009-07-17 09:11:31 +10:00

58 lines
1.3 KiB
C

/* Function to determine if a thread group is single threaded or not
*
* Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)
* - Derived from security/selinux/hooks.c
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public Licence
* as published by the Free Software Foundation; either version
* 2 of the Licence, or (at your option) any later version.
*/
#include <linux/sched.h>
/*
* Returns true if the task does not share ->mm with another thread/process.
*/
bool current_is_single_threaded(void)
{
struct task_struct *task = current;
struct mm_struct *mm = task->mm;
struct task_struct *p, *t;
bool ret;
if (atomic_read(&task->signal->live) != 1)
return false;
if (atomic_read(&mm->mm_users) == 1)
return true;
ret = false;
rcu_read_lock();
for_each_process(p) {
if (unlikely(p->flags & PF_KTHREAD))
continue;
if (unlikely(p == task->group_leader))
continue;
t = p;
do {
if (unlikely(t->mm == mm))
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();
return ret;
}