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    SubjectRe: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
    On Sun, Apr 07, 2019 at 03:46:13PM +0000, Joel Fernandes wrote:
    > 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.

    Hard to argue given that I just deleted include/asm-generic/vmlinux.lds.h,
    touched kernel/rcu/rcutorture.c, and rebuilt the corresponding .ko
    without errors. ;-)

    Hmmm... Is there some way to place a section into a read-only page,
    for example, tagged onto the text section for that module? That would
    get rid of a class of bugs, if nothing else.

    > 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.

    Please do resend as a formal patch with the above in the commit log.
    I doubt that I am the only one needing a bit of module-build education!
    And thank you for providing that education, by the way!

    > > And may I have your Tested-by?
    >
    > Absolutely, please do and thanks!

    Done, and thank you for giving it a go!
    Thanx, Paul

    > - 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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