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SubjectRe: [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@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 = .; \
> } \
> \
> .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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