# Re: \[PATCH v3 10/14\] locking/percpu-rwsem: Add percpu\_is\_write\_locked() and percpu\_is\_read\_locked()

- From: Marco Elver \<elver@google.com\>
- Date: 2022-08-29 06:01:44 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2022/08/1911901
- Thread: https://ratatoskr.run/linuxppc-dev/2022/07/1845740/t.md

```text
On Wed, 17 Aug 2022 at 14:48, Peter Zijlstra [ref] wrote:
> On Mon, Jul 04, 2022 at 05:05:10PM +0200, Marco Elver wrote:
> > +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> > +{
> > +     return per_cpu_sum(*sem->read_count) != 0;
> > +}
> > +EXPORT_SYMBOL_GPL(percpu_is_read_locked);
>
> I don't think this is correct; read_count can have spurious increments.
>
> If we look at __percpu_down_read_trylock(), it does roughly something
> like this:
>
>         this_cpu_inc(*sem->read_count);
>         smp_mb();
>         if (!sem->block)
>                 return true;
>         this_cpu_dec(*sem->read_count);
>         return false;
>
> So percpu_is_read_locked() needs to ensure the read_count is non-zero
> *and* that block is not set.

I shall go and fix. v4 incoming (if more comments before that, please shout).

> That said; I really dislike the whole _is_locked family with a passion.
> Let me try and figure out what you need this for.

As in the other email, it's for the dbg_*() functions for kgdb's
benefit (avoiding deadlock if kgdb wants a breakpoint, while we're in
the process of handing out a breakpoint elsewhere and have the locks
taken).

Thanks,
-- Marco
```