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

["Paul E. McKenney" <paulmck@linux.ibm.com>]

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
>