On 07/09, Oleg Nesterov wrote:
>
> 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.
To simplify the review, below is the code after the patch.
Oleg.
/*
* Returns true if the task does not share ->mm with another thread/process.
*/
bool is_current_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;
}
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