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Date:   Sat, 6 Apr 2019 13:33:27 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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>,
        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 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@...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!
> 
> 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@...ux.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@...icios.com>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.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 = .;				\
>  	}								\
>  									\
>  	.rodata1          : AT(ADDR(.rodata1) - LOAD_OFFSET) {		\
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 5bf5dcd91009..921443a026dd 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -21,6 +21,7 @@
>  #include <linux/rbtree_latch.h>
>  #include <linux/error-injection.h>
>  #include <linux/tracepoint-defs.h>
> +#include <linux/srcu.h>
>  
>  #include <linux/percpu.h>
>  #include <asm/module.h>
> @@ -448,6 +449,10 @@ struct module {
>  	unsigned int num_tracepoints;
>  	tracepoint_ptr_t *tracepoints_ptrs;
>  #endif
> +#ifdef CONFIG_TREE_SRCU
> +	unsigned int num_srcu_structs;
> +	struct srcu_struct **srcu_struct_ptrs;
> +#endif
>  #ifdef CONFIG_BPF_EVENTS
>  	unsigned int num_bpf_raw_events;
>  	struct bpf_raw_event_map *bpf_raw_events;
> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> index 7f7c8c050f63..93685a9f3b4c 100644
> --- a/include/linux/srcutree.h
> +++ b/include/linux/srcutree.h
> @@ -120,9 +120,17 @@ struct srcu_struct {
>   *
>   * See include/linux/percpu-defs.h for the rules on per-CPU variables.
>   */
> -#define __DEFINE_SRCU(name, is_static)					\
> -	static DEFINE_PER_CPU(struct srcu_data, name##_srcu_data);\
> -	is_static struct srcu_struct name = __SRCU_STRUCT_INIT(name, name##_srcu_data)
> +#ifdef MODULE
> +# define __DEFINE_SRCU(name, is_static)					\
> +	is_static struct srcu_struct name;				\
> +	struct srcu_struct *__srcu_struct_##name			\
> +		__attribute__((section("___srcu_struct_ptrs"))) = &name

This can be replaced by:
__section("__srcu_struct_ptrs") = &name;

> +#else
> +# define __DEFINE_SRCU(name, is_static)					\
> +	static DEFINE_PER_CPU(struct srcu_data, name##_srcu_data);	\
> +	is_static struct srcu_struct name =				\
> +		__SRCU_STRUCT_INIT(name, name##_srcu_data)
> +#endif
>  #define DEFINE_SRCU(name)		__DEFINE_SRCU(name, /* not static */)
>  #define DEFINE_STATIC_SRCU(name)	__DEFINE_SRCU(name, static)
>  
> diff --git a/kernel/module.c b/kernel/module.c
> index 0b9aa8ab89f0..524da609c884 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3093,6 +3093,11 @@ static int find_module_sections(struct module *mod, struct load_info *info)
>  					     sizeof(*mod->tracepoints_ptrs),
>  					     &mod->num_tracepoints);
>  #endif
> +#ifdef CONFIG_TREE_SRCU
> +	mod->srcu_struct_ptrs = section_objs(info, "___srcu_struct_ptrs",
> +					     sizeof(*mod->srcu_struct_ptrs),
> +					     &mod->num_srcu_structs);
> +#endif
>  #ifdef CONFIG_BPF_EVENTS
>  	mod->bpf_raw_events = section_objs(info, "__bpf_raw_tp_map",
>  					   sizeof(*mod->bpf_raw_events),
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 9b761e546de8..ac0f6f0a8916 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1310,3 +1310,70 @@ void __init srcu_init(void)
>  		queue_work(rcu_gp_wq, &ssp->work.work);
>  	}
>  }
> +
> +#ifdef CONFIG_MODULES
> +
> +/* Initialize any global-scope srcu_struct structures used by this module. */
> +static int srcu_module_coming(struct module *mod)
> +{
> +	int i;
> +	struct srcu_struct **sspp = mod->srcu_struct_ptrs;
> +	int ret;
> +
> +	for (i = 0; i < mod->num_srcu_structs; i++) {
> +		ret = init_srcu_struct(*(sspp++));
> +		if (WARN_ON_ONCE(ret))
> +			return ret;
> +	}
> +	return 0;
> +}
> +
> +/* Clean up any global-scope srcu_struct structures used by this module. */
> +static void srcu_module_going(struct module *mod)
> +{
> +	int i;
> +	struct srcu_struct **sspp = mod->srcu_struct_ptrs;
> +
> +	for (i = 0; i < mod->num_srcu_structs; i++)
> +		cleanup_srcu_struct(*(sspp++));
> +}
> +
> +/* Handle one module, either coming or going. */
> +static int srcu_module_notify(struct notifier_block *self,
> +			      unsigned long val, void *data)
> +{
> +	struct module *mod = data;
> +	int ret = 0;
> +
> +	switch (val) {
> +	case MODULE_STATE_COMING:
> +		ret = srcu_module_coming(mod);
> +		break;
> +	case MODULE_STATE_LIVE:
> +		break;
> +	case MODULE_STATE_GOING:
> +		srcu_module_going(mod);
> +		break;
> +	case MODULE_STATE_UNFORMED:
> +		break;

The unused cases can be put under default or even clubbed thus saving a line
but if you prefer to keep them.like this that is fine.

It looks good to me but I will test it out more later today. Thanks!

- Joel


> +	return ret;
> +}
> +
> +static struct notifier_block srcu_module_nb = {
> +	.notifier_call = srcu_module_notify,
> +	.priority = 0,
> +};
> +
> +static __init int init_srcu_module_notifier(void)
> +{
> +	int ret;
> +
> +	ret = register_module_notifier(&srcu_module_nb);
> +	if (ret)
> +		pr_warn("Failed to register srcu module notifier\n");
> +	return ret;
> +}
> +late_initcall(init_srcu_module_notifier);
> +
> +#endif /* #ifdef CONFIG_MODULES */
> 

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