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SubjectRe: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
On Sun, Apr 07, 2019 at 06:59:37AM -0700, Paul E. McKenney wrote:
> On Sun, Apr 07, 2019 at 06:39:41AM -0700, Paul E. McKenney wrote:
> > On Sat, Apr 06, 2019 at 07:06:13PM -0400, Joel Fernandes wrote:
>
> [ . . . ]
>
> > > > 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?
>
> And to answer the other question, in the case where I am suffering from
> excessive optimism, it should be a separate commit. Please see below
> for the updated original commit thus far.
>

Actually the vmlinux.lds.h file is unused for module building. For ex, if you
delete include/asm-generic/vmlinux.lds.h , then you can still build
rcutorture.ko. Did I miss something obvious? In that case the vmlinux.lds.h
are not needed since the __section annotations automatically place the srcu
structs in a separate section.

Let me know if you would like me to send a patch separately, or if the
appended patch for the same in my previous email suffices.

> And may I have your Tested-by?

Absolutely, please do and thanks!

- Joel


> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit a365bb5f6eafb220a1448674054b05c250829313
> 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>
> [ paulmck: Use __section() and use "default" in srcu_module_notify()'s
> "switch" statement as suggested by Joel Fernandes. ]
> 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..8af1824c46a8 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 \
> + __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..2ded2614a2f4 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1310,3 +1310,68 @@ 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_GOING:
> + srcu_module_going(mod);
> + break;
> + default:
> + break;
> + }
> + 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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