On Tue, Apr 02, 2019 at 11:34:07AM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 2, 2019, at 11:23 AM, paulmck paulmck@xxxxxxxxxxxxx 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?
>
> I was more thinking that under #ifdef MODULE, both DEFINE_SRCU and
> DEFINE_STATIC_SRCU could append data in a dedicated section. module.c
> would additionally lookup that section on module load, and deal with
> those statically defined SRCU entries as if they were dynamically
> allocated ones. It would of course cleanup those resources on module
> unload.
>
> Am I missing some subtlety there ?
If I understand you correctly, that is actually what is already done. The
size of this dedicated section is currently set by PERCPU_MODULE_RESERVE,
and the additions of DEFINE{_STATIC}_SRCU() in modules was requiring that
this to be increased frequently. That led to a request that something
be done, in turn leading to this patch series.
I don't see a way around this short of changing module loading to do
alloc_percpu() and then updating the relocation based on this result.
Which would admittedly be far more convenient. I was assuming that
this would be difficult due to varying CPU offsets or the like.
But if it can be done reasonably, it would be quite a bit nicer than
forcing dynamic allocation in cases where it is not otherwise needed.
Thanx, Paul
> Thanks,
>
> Mathieu
>
>
> >
> > Thanx, Paul
> >
> >> Thanks,
> >>
> >> Mathieu
> >>
> >>
> >> >
> >> > This series consist of the following:
> >> >
> >> > 1. Dynamically allocate dax_srcu.
> >> >
> >> > 2. Dynamically allocate drm_unplug_srcu.
> >> >
> >> > 3. Dynamically allocate kfd_processes_srcu.
> >> >
> >> > These build and have been subjected to 0day testing, but might also need
> >> > testing by someone having the requisite hardware.
> >> >
> >> > Thanx, Paul
> >> >
> >> > ------------------------------------------------------------------------
> >> >
> >> > drivers/dax/super.c | 10 +++++-
> >> > drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd.c | 5 +++
> >> > drivers/gpu/drm/amd/amdkfd/kfd_process.c | 2 -
> >> > drivers/gpu/drm/drm_drv.c | 8 ++++
> >> > include/linux/srcutree.h | 19 +++++++++--
> >> > kernel/rcu/rcuperf.c | 40 +++++++++++++++++++-----
> >> > kernel/rcu/rcutorture.c | 48 +++++++++++++++++++++--------
> >> > 7 files changed, 105 insertions(+), 27 deletions(-)
> >>
> >> --
> >> Mathieu Desnoyers
> >> EfficiOS Inc.
> >> http://www.efficios.com
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>
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