Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access

Joel Fernandes
2020-05-20T18:44:28Z

On Wed, May 20, 2020 at 08:35:29PM +0200, Peter Zijlstra wrote:
> On Wed, May 20, 2020 at 08:28:00PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2020-05-20 13:42:59 [-0400], Joel Fernandes wrote:
> > > Hi Sebastian,
> > Hi Joel,
> >
> > > For pointer stability, can we just use get_local_ptr() and put_local_ptr()
> > > instead of adding an extra lock? This keeps the pointer stable while keeping
> > > the section preemptible on -rt. And we already have a lock in rcu_data, I
> > > prefer not to add another lock if possible.
> >
> > What is this get_local_ptr() doing? I can't find it anywhere…
>
> I suspect it is ({ preempt_disable(); this_cpu_ptr(ptr); }), or
> something along those lines.
>
> But yeah, I can't find it either.

I actually found it in RT 4.4 kernel, I thought this was also on newer RT
kernels as well (is that not true anymore?). But yes it was exactly what
Peter said.

In 4.4 RT there's code similar to:

#ifdef CONFIG_PREEMPT_RT_FULL

# define get_local_ptr(var) ({ \
migrate_disable(); \
this_cpu_ptr(var); })

# define put_local_ptr(var) do { \
(void)(var); \
migrate_enable(); \
} while (0)

#else

#define get_local_ptr(var) get_cpu_ptr(var)
#define put_local_ptr(var) put_cpu_ptr(var)

#endif

thanks,

- Joel