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

[Paul E. McKenney]

On Tue, Apr 02, 2019 at 02:40:54PM -0400, Joel Fernandes wrote:
> 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@xxxxxxxxxxxxx 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?

The percpu/module reservation discussed in this thread:

https://lore.kernel.org/lkml/c72402f2-967e-cd56-99d8-9139c9e7f267@xxxxxxxxxx/T/#mbcacf60ddc0b3fd6e831a3ea71efc90da359a3bf

For non-modules, global per-CPU variables are statically allocated.
For modules, they must be dynamically allocated at modprobe time, and
their size is set by PERCPU_MODULE_RESERVE.

							Thanx, Paul