Re: [PATCH] rework/fix is_single_threaded()

[Oleg Nesterov]

On 06/22, Andrew Morton wrote:
>
> hm, that's still all a bit marginal/waffly.
>
> >
> > > From: Oleg Nesterov <oleg@xxxxxxxxxx>
> > >
> > > - Fix the comment, is_single_threaded(p) actually means that nobody shares
> > >   ->mm with p.
> > >
> > >   I think this helper should be renamed, and it should not have arguments.
> > >   With or without this patch it must not be used unless p == current,
> > >   otherwise we can't safely use p->signal or p->mm.
> > >
> > > - "if (atomic_read(&p->signal->count) != 1)" is not right when we have a
> > >   zombie group leader, use signal->live instead.
> > >
> > > - Add PF_KTHREAD check to skip kernel threads which may borrow p->mm,
> > >   otherwise we can return the wrong "false".
> > >
> > > - Use for_each_process() instead of do_each_thread(), all threads must use
> > >   the same ->mm.
> > >
> > > - Use down_write(mm->mmap_sem) + rcu_read_lock() instead of tasklist_lock
> > >   to iterate over the process list. If there is another CLONE_VM process
> > >   it can't pass exit_mm() which takes the same mm->mmap_sem. We can miss
> > >   a freshly forked CLONE_VM task, but this doesn't matter because we must
> > >   see its parent and return false.
>
> Maybe we should do the locking change in a separate and subsequent
> patch?

Sure, I can split these changes. Or we can just forget about this patch.


But what is the problem with this patch?

David, do you still dislike ->mmap_sem? I didn't see other objections,
and again, imho tasklist_lock is worse.

Oleg.

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