Re: Protection of critical section in PREEMPT_RT

[Stanislav Meduna <stano@meduna.org>]

On 22.01.2013 04:29, Jacky Lam wrote:

> I have a kernel driver needed to do a indirect register access
> to a hardware interface. In past, I use spinlock to protect
> those code. After turning on PREEMPT_RT, the driver doesn't
> work anymore.

I'm not an expert but AFAIK the spinlock still does the protection,
it just can be preempted. If your driver does not work, there
is either a timing issue as well (i.e. you have time constraints
that are not met when the spinlock is preempted) or there is
a bug that is triggered by the preemption (such as unprotected
access in the code that preempts your spinlock).

> I know spinlock is now preemptible, but how can I protect a "really"
> critical section in PREEMPT_RT? If it is a bad design, could give me
> some tutorial/readings on how to modify my driver?

https://rt.wiki.kernel.org/index.php/RT_PREEMPT_HOWTO

says

Critical sections protected by i.e. spinlock_t and rwlock_t are now
preemptible. The creation of non-preemptible sections (in kernel) is
still possible with raw_spinlock_t (same APIs like spinlock_t)

However, try to identify the real culprit first.

--
                                          Stano