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SubjectRe: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
On Sat, Apr 06, 2019 at 07:06:13PM -0400, Joel Fernandes wrote:
> On Fri, Apr 05, 2019 at 04:28:35PM -0700, Paul E. McKenney wrote:
> > On Wed, Apr 03, 2019 at 09:20:39AM -0700, Paul E. McKenney wrote:
> > > On Wed, Apr 03, 2019 at 10:27:42AM -0400, Mathieu Desnoyers wrote:
> > > > ----- On Apr 3, 2019, at 9:32 AM, paulmck paulmck@linux.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@linux.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@linux.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!
> >
> > Please see below for an untested shot.
> >
> > The original commits posted in this series are still available within
> > the -srcu tree at branch srcunomod.2019.04.05a. Yes, I am a digital
> > packrat. Why do you ask?
> >
> > Thoughts? Or more accurately, given that this is the first time I
> > have used linker sections, what did I mess up?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit e24a0dab1414c563bb96bcb28d5963c9df18b1e8
> > Author: Paul E. McKenney <paulmck@linux.ibm.com>
> > Date: Fri Apr 5 16:15:00 2019 -0700
> >
> > srcu: Allocate per-CPU data for DEFINE_SRCU() in modules
> >
> > Adding DEFINE_SRCU() or DEFINE_STATIC_SRCU() to a loadable module requires
> > that the size of the reserved region be increased, which is not something
> > we want to be doing all that often. One approach would be to require
> > that loadable modules define an srcu_struct and invoke init_srcu_struct()
> > from their module_init function and cleanup_srcu_struct() from their
> > module_exit function. However, this is more than a bit user unfriendly.
> >
> > This commit therefore creates an ___srcu_struct_ptrs linker section,
> > and pointers to srcu_struct structures created by DEFINE_SRCU() and
> > DEFINE_STATIC_SRCU() within a module are placed into that module's
> > ___srcu_struct_ptrs section. The required init_srcu_struct() and
> > cleanup_srcu_struct() functions are then automatically invoked as needed
> > when that module is loaded and unloaded, thus allowing modules to continue
> > to use DEFINE_SRCU() and DEFINE_STATIC_SRCU() while avoiding the need
> > to increase the size of the reserved region.
> >
> > Many of the algorithms and some of the code was cheerfully cherry-picked
> > from other code making use of linker sections, perhaps most notably from
> > tracepoints. All bugs are nevertheless the sole property of the author.
> >
> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index f8f6f04c4453..c2d919a1566e 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -338,6 +338,10 @@
> > KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \
> > __stop___tracepoints_ptrs = .; \
> > *(__tracepoints_strings)/* Tracepoints: strings */ \
> > + . = ALIGN(8); \
> > + __start___srcu_struct = .; \
> > + *(___srcu_struct_ptrs) \
> > + __end___srcu_struct = .; \
> > } \
>
> This vmlinux linker modification is not needed. I tested without it and srcu
> torture works fine with rcutorture built as a module. Putting further prints
> in kernel/module.c verified that the kernel is able to find the srcu structs
> just fine. You could squash the below patch into this one or apply it on top
> of the dev branch.

Good point, given that otherwise FORTRAN named common blocks would not
work.

But isn't one advantage of leaving that stuff in the RO_DATA_SECTION()
macro that it can be mapped read-only? Or am I suffering from excessive
optimism?

Thanx,Paul

> Thanks!
>
> ---8<-----------------------
>
> >From 369ad090f706ce8e1facdd18eb10828b5f7e2b72 Mon Sep 17 00:00:00 2001
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> Date: Sat, 6 Apr 2019 18:57:17 -0400
> Subject: [PATCH] srcu: Remove unused vmlinux srcu linker entries
>
> The SRCU for modules optimization introduced vmlinux linker entries
> which is unused since it applies only to the built-in vmlinux. So remove
> it to prevent any space usage due to the 8 byte alignment.
>
> Tested with SRCU torture_type and rcutorture.
>
> Cc: kernel-team@android.com
> Cc: paulmck@linux.vnet.ibm.com
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> include/asm-generic/vmlinux.lds.h | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index c2d919a1566e..f8f6f04c4453 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -338,10 +338,6 @@
> KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \
> __stop___tracepoints_ptrs = .; \
> *(__tracepoints_strings)/* Tracepoints: strings */ \
> - . = ALIGN(8); \
> - __start___srcu_struct = .; \
> - *(___srcu_struct_ptrs) \
> - __end___srcu_struct = .; \
> } \
> \
> .rodata1 : AT(ADDR(.rodata1) - LOAD_OFFSET) { \
> --
> 2.21.0.392.gf8f6787159e-goog
>

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