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Message-Id: <20190403162039.GA14111@linux.ibm.com>
Date: Wed, 3 Apr 2019 09:20:39 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: rcu <rcu@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
dipankar <dipankar@...ibm.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Josh Triplett <josh@...htriplett.org>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
rostedt <rostedt@...dmis.org>,
David Howells <dhowells@...hat.com>,
Eric Dumazet <edumazet@...gle.com>,
fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
"Joel Fernandes, Google" <joel@...lfernandes.org>,
linux-nvdimm <linux-nvdimm@...ts.01.org>,
dri-devel <dri-devel@...ts.freedesktop.org>,
amd-gfx <amd-gfx@...ts.freedesktop.org>
Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
On Wed, Apr 03, 2019 at 10:27:42AM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 3, 2019, at 9:32 AM, paulmck paulmck@...ux.ibm.com wrote:
>
> > On Tue, Apr 02, 2019 at 11:34:07AM -0400, Mathieu Desnoyers wrote:
> >> ----- On Apr 2, 2019, at 11:23 AM, paulmck paulmck@...ux.ibm.com wrote:
> >>
> >> > On Tue, Apr 02, 2019 at 11:14:40AM -0400, Mathieu Desnoyers wrote:
> >> >> ----- On Apr 2, 2019, at 10:28 AM, paulmck paulmck@...ux.ibm.com 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 think we are not expressing quite the same idea.
>
> AFAIU, yours is to have DEFINE*_SRCU directly define per-cpu data within modules,
> which ends up using percpu module reserved memory.
>
> My idea is to make DEFINE*_SRCU have a different behavior under #ifdef MODULE.
> It could emit a _global variable_ (_not_ per-cpu) within a new section. That
> section would then be used by module init/exit code to figure out what "srcu
> descriptors" are present in the modules. It would therefore rely on dynamic
> allocation for those, therefore removing the need to involve the percpu module
> reserved pool at all.
>
> >
> > 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.
>
> Hopefully my explanation above helps clear out what I have in mind.
>
> You can find similar tricks performed by include/linux/tracepoint.h:
>
> #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> {
> return offset_to_ptr(p);
> }
>
> #define __TRACEPOINT_ENTRY(name) \
> asm(" .section \"__tracepoints_ptrs\", \"a\" \n" \
> " .balign 4 \n" \
> " .long __tracepoint_" #name " - . \n" \
> " .previous \n")
> #else
> static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> {
> return *p;
> }
>
> #define __TRACEPOINT_ENTRY(name) \
> static tracepoint_ptr_t __tracepoint_ptr_##name __used \
> __attribute__((section("__tracepoints_ptrs"))) = \
> &__tracepoint_##name
> #endif
>
> [...]
>
> #define DEFINE_TRACE_FN(name, reg, unreg) \
> static const char __tpstrtab_##name[] \
> __attribute__((section("__tracepoints_strings"))) = #name; \
> struct tracepoint __tracepoint_##name \
> __attribute__((section("__tracepoints"), used)) = \
> { __tpstrtab_##name, STATIC_KEY_INIT_FALSE, reg, unreg, NULL };\
> __TRACEPOINT_ENTRY(name);
>
> And kernel/module.c:
>
> find_module_sections():
>
> #ifdef CONFIG_TRACEPOINTS
> mod->tracepoints_ptrs = section_objs(info, "__tracepoints_ptrs",
> sizeof(*mod->tracepoints_ptrs),
> &mod->num_tracepoints);
> #endif
>
> And kernel/tracepoint.c:tracepoint_module_notify() for the module coming/going
> notifier.
>
> Basically you would want to have your own structure within your own section of
> the module which describes the srcu domain, and have a module coming/going
> notifier responsible for dynamically allocating the srcu domain on "coming", and
> doing a srcu barrier and cleanup the domain on "going".
Ah, sounds like an excellent approach! I will give it a shot, thank you!
Thanx, Paul
> Thanks,
>
> Mathieu
>
>
> >
> > 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
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>
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