srcu: Use static init for statically allocated in-module srcu_struct

Further shrinking the srcu_struct structure is eased by requiring
that in-module srcu_struct structures rely more heavily on static
initialization.  In particular, this preserves the property that
a module-load-time srcu_struct initialization can fail only due
to memory-allocation failure of the per-CPU srcu_data structures.
It might also slightly improve robustness by keeping the number of memory
allocations that must succeed down percpu_alloc() call.

This is in preparation for splitting an srcu_usage structure out
of the srcu_struct structure.

[ paulmck: Fold in qiang1.zhang@intel.com feedback. ]

Cc: Christoph Hellwig <hch@lst.de>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/srcutree.h

@@ -108,15 +108,24 @@ struct srcu_struct {
 #define SRCU_STATE_SCAN1	1
 #define SRCU_STATE_SCAN2	2
 
-#define __SRCU_STRUCT_INIT(name, pcpu_name)							\
-{												\
-	.sda = &pcpu_name,									\
+#define __SRCU_STRUCT_INIT_COMMON(name)								\
 	.lock = __SPIN_LOCK_UNLOCKED(name.lock),						\
 	.srcu_gp_seq_needed = -1UL,								\
 	.work = __DELAYED_WORK_INITIALIZER(name.work, NULL, 0),					\
-	__SRCU_DEP_MAP_INIT(name)								\
+	__SRCU_DEP_MAP_INIT(name)
+
+#define __SRCU_STRUCT_INIT_MODULE(name)								\
+{												\
+	__SRCU_STRUCT_INIT_COMMON(name)								\
 }
 
+#define __SRCU_STRUCT_INIT(name, pcpu_name)							\
+{												\
+	.sda = &pcpu_name,									\
+	__SRCU_STRUCT_INIT_COMMON(name)								\
+}
+
+
 /*
  * Define and initialize a srcu struct at build time.
  * Do -not- call init_srcu_struct() nor cleanup_srcu_struct() on it.
@@ -138,7 +147,7 @@ struct srcu_struct {
  */
 #ifdef MODULE
 # define __DEFINE_SRCU(name, is_static)								\
-	is_static struct srcu_struct name;							\
+	is_static struct srcu_struct name = __SRCU_STRUCT_INIT_MODULE(name);			\
 	extern struct srcu_struct * const __srcu_struct_##name;					\
 	struct srcu_struct * const __srcu_struct_##name						\
 		__section("___srcu_struct_ptrs") = &name

kernel/rcu/srcutree.c

@@ -1873,13 +1873,14 @@ void __init srcu_init(void)
 static int srcu_module_coming(struct module *mod)
 {
 	int i;
+	struct srcu_struct *ssp;
 	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;
+		ssp = *(sspp++);
+		ssp->sda = alloc_percpu(struct srcu_data);
+		if (WARN_ON_ONCE(!ssp->sda))
+			return -ENOMEM;
 	}
 	return 0;
 }
@@ -1888,10 +1889,16 @@ static int srcu_module_coming(struct module *mod)
 static void srcu_module_going(struct module *mod)
 {
 	int i;
+	struct srcu_struct *ssp;
 	struct srcu_struct **sspp = mod->srcu_struct_ptrs;
 
-	for (i = 0; i < mod->num_srcu_structs; i++)
-		cleanup_srcu_struct(*(sspp++));
+	for (i = 0; i < mod->num_srcu_structs; i++) {
+		ssp = *(sspp++);
+		if (!rcu_seq_state(smp_load_acquire(&ssp->srcu_gp_seq_needed)) &&
+		    !WARN_ON_ONCE(!ssp->sda_is_static))
+			cleanup_srcu_struct(ssp);
+		free_percpu(ssp->sda);
+	}
 }
 
 /* Handle one module, either coming or going. */