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

[Paul E. McKenney]

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.

And may I have your Tested-by?

							Thanx, Paul

------------------------------------------------------------------------

commit a365bb5f6eafb220a1448674054b05c250829313
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
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@xxxxxxxxxxxx>
    [ 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@xxxxxxxxxxxxx>

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 */