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SubjectRe: [PATCH] rework/fix is_single_threaded()
On 04/16, Oleg Nesterov wrote:
>
> Suppose we have a process P which shares ->mm with "task" (the argument), so
> we should return "false".
>
> P does clone(CLONE_VM) and exits. rcu_read_lock() can't guarantee we will
> see the new task with the same ->mm. And without ->mmap_sem P can call
> exit_mm() and set P->mm = NULL.
>
> Hmm. But we can just add a barrier?
>
> bool is_single_threaded(struct task_struct *task)
> {
> 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. Perhaps it had the same ->mm ?
> If t has forked CLONE_VM task and called exit_mm(),
> make sure next_thread() or for_each_process()->next_task()
> will see it.
> !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> */
> smp_rmb();

Sorry, forgot to mention...

But what if P does clone(CLONE_VM), exits, and for_each_process/while_each_thread
doesn't see it? IOW, what if we already see the result of list_del_rcu() ?

I think, in that case we must also see the result of clone()->list_add_tail_rcu()
which has a barrier, so we are safe.

Hmm. I feel this all has a simpler explanation, or I missed something...

Oleg.



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