Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules

[Joel Fernandes <joel-QYYGw3jwrUn5owFQY34kdNi2O/JbrIOy@public.gmane.org>]

On Tue, Apr 02, 2019 at 08:23:34AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 02, 2019 at 11:14:40AM -0400, Mathieu Desnoyers wrote:
> > ----- On Apr 2, 2019, at 10:28 AM, paulmck paulmck-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org wrote:
> > 
> > > Hello!
> > > 
> > > This series prohibits use of DEFINE_SRCU() and DEFINE_STATIC_SRCU()
> > > by loadable modules.  The reason for this prohibition is the fact
> > > that using these two macros within modules requires that the size of
> > > the reserved region be increased, which is not something we want to
> > > be doing all that often.  Instead, loadable modules should define an
> > > srcu_struct and invoke init_srcu_struct() from their module_init function
> > > and cleanup_srcu_struct() from their module_exit function.  Note that
> > > modules using call_srcu() will also need to invoke srcu_barrier() from
> > > their module_exit function.
> > 
> > This arbitrary API limitation seems weird.
> > 
> > Isn't there a way to allow modules to use DEFINE_SRCU and DEFINE_STATIC_SRCU
> > while implementing them with dynamic allocation under the hood ?
> 
> Although call_srcu() already has initialization hooks, some would
> also be required in srcu_read_lock(), and I am concerned about adding
> memory allocation at that point, especially given the possibility
> of memory-allocation failure.  And the possibility that the first
> srcu_read_lock() happens in an interrupt handler or similar.
> 
> Or am I missing a trick here?

Hi Paul,

Which 'reserved region' are you referring to? Isn't this region also
used for non-module cases in which case the same problem applies to
non-modules?

thanks!

 - Joel