Re: [PATCH] change current_is_single_threaded() to use for_each_thread()

[Oleg Nesterov <oleg@redhat.com>]

On 10/07, David Howells wrote:
>
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> > Change current_is_single_threaded() to use for_each_thread() rather
> > than deprecated while_each_thread().
>
> Wouldn't the old way be more efficient, given there's always going to be at
> least one thread?

Yes, unfortunately. Before this change (in the likely case) we never
do next_thread(), after this patch we always do list_first_entry().

> Granted, it's not much of an efficiency boost...

Yes, and while_each_thread() is buggy. See the changelog in 0c740d0afc
"introduce for_each_thread() to replace the buggy while_each_thread().

Yes, the race is almost purely theoretical, but still.

To clarify, we had some "real" bugs fixed by s/while_each_thread/
for_each_thread/, but only because while_each_thread() was used wrongly.
for_each_thread() is more safe, it only needs the stable task_struct.

Oleg.