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