[PATCH] srcu: faster srcu gp seq wrap-around

[JP Kobryn <inwardvessel@xxxxxxxxx>]

Using a higher value for the initial gp sequence counter values allows for
wrapping to occur faster. It can help with surfacing any issues that may be
happening as a result of the wrap around.

Signed-off-by: JP Kobryn <inwardvessel@xxxxxxxxx>
---
 include/linux/rcupdate.h |  3 +++
 include/linux/srcutree.h | 15 ++++++++++++++-
 kernel/rcu/rcu.h         |  3 ---
 kernel/rcu/srcutree.c    |  6 ++++--
 4 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index dfd2399f2cde..c148674b9e09 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -35,6 +35,9 @@
 #define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
 #define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))
 
+#define RCU_SEQ_CTR_SHIFT    2
+#define RCU_SEQ_STATE_MASK   ((1 << RCU_SEQ_CTR_SHIFT) - 1)
+
 /* Exported common interfaces */
 void call_rcu(struct rcu_head *head, rcu_callback_t func);
 void rcu_barrier_tasks(void);
diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 8f3f72480e78..ed57598394de 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -129,10 +129,23 @@ struct srcu_struct {
 #define SRCU_STATE_SCAN1	1
 #define SRCU_STATE_SCAN2	2
 
+/*
+ * Values for initializing gp sequence fields. Higher values allow wrap arounds to
+ * occur earlier.
+ * The second value with state is useful in the case of static initialization of
+ * srcu_usage where srcu_gp_seq_needed is expected to have some state value in its
+ * lower bits (or else it will appear to be already initialized within
+ * the call check_init_srcu_struct()).
+ */
+#define SRCU_GP_SEQ_INITIAL_VAL ((0UL - 100UL) << RCU_SEQ_CTR_SHIFT)
+#define SRCU_GP_SEQ_INITIAL_VAL_WITH_STATE (SRCU_GP_SEQ_INITIAL_VAL - 1)
+
 #define __SRCU_USAGE_INIT(name)									\
 {												\
 	.lock = __SPIN_LOCK_UNLOCKED(name.lock),						\
-	.srcu_gp_seq_needed = -1UL,								\
+	.srcu_gp_seq = SRCU_GP_SEQ_INITIAL_VAL,							\
+	.srcu_gp_seq_needed = SRCU_GP_SEQ_INITIAL_VAL_WITH_STATE,				\
+	.srcu_gp_seq_needed_exp = SRCU_GP_SEQ_INITIAL_VAL,					\
 	.work = __DELAYED_WORK_INITIALIZER(name.work, NULL, 0),					\
 }
 
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 38238e595a61..2bfed9855d67 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -54,9 +54,6 @@
  *					grace-period sequence number.
  */
 
-#define RCU_SEQ_CTR_SHIFT	2
-#define RCU_SEQ_STATE_MASK	((1 << RCU_SEQ_CTR_SHIFT) - 1)
-
 /* Low-order bit definition for polled grace-period APIs. */
 #define RCU_GET_STATE_COMPLETED	0x1
 
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index bc4b58b0204e..2209dd0589de 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -247,7 +247,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	mutex_init(&ssp->srcu_sup->srcu_cb_mutex);
 	mutex_init(&ssp->srcu_sup->srcu_gp_mutex);
 	ssp->srcu_idx = 0;
-	ssp->srcu_sup->srcu_gp_seq = 0;
+	ssp->srcu_sup->srcu_gp_seq = SRCU_GP_SEQ_INITIAL_VAL;
 	ssp->srcu_sup->srcu_barrier_seq = 0;
 	mutex_init(&ssp->srcu_sup->srcu_barrier_mutex);
 	atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0);
@@ -258,7 +258,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	if (!ssp->sda)
 		goto err_free_sup;
 	init_srcu_struct_data(ssp);
-	ssp->srcu_sup->srcu_gp_seq_needed_exp = 0;
+	ssp->srcu_sup->srcu_gp_seq_needed_exp = SRCU_GP_SEQ_INITIAL_VAL;
 	ssp->srcu_sup->srcu_last_gp_end = ktime_get_mono_fast_ns();
 	if (READ_ONCE(ssp->srcu_sup->srcu_size_state) == SRCU_SIZE_SMALL && SRCU_SIZING_IS_INIT()) {
 		if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC))
@@ -267,6 +267,8 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	}
 	ssp->srcu_sup->srcu_ssp = ssp;
 	smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed, 0); /* Init done. */
+	smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed,
+		SRCU_GP_SEQ_INITIAL_VAL); /* Init done. */
 	return 0;
 
 err_free_sda:
-- 
2.45.2