[PATCH RFC 09/16] prcu: Implement prcu_barrier() API

[lianglihao]

From: Lihao Liang <lianglihao@xxxxxxxxxx>

This is PRCU's counterpart of RCU's rcu_barrier() API.

Reviewed-by: Heng Zhang <heng.z@xxxxxxxxxx>
Signed-off-by: Lihao Liang <lianglihao@xxxxxxxxxx>
---
 include/linux/prcu.h |  7 ++++++
 kernel/rcu/prcu.c    | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+)

diff --git a/include/linux/prcu.h b/include/linux/prcu.h
index 4e7d5d65..cce967fd 100644
--- a/include/linux/prcu.h
+++ b/include/linux/prcu.h
@@ -5,6 +5,7 @@
 #include <linux/types.h>
 #include <linux/mutex.h>
 #include <linux/wait.h>
+#include <linux/completion.h>
 
 #define CONFIG_PRCU
 
@@ -32,6 +33,7 @@ struct prcu_local_struct {
 	unsigned int online;
 	unsigned long long version;
 	unsigned long long cb_version;
+	struct rcu_head barrier_head;
 	struct prcu_cblist cblist;
 };
 
@@ -39,8 +41,11 @@ struct prcu_struct {
 	atomic64_t global_version;
 	atomic64_t cb_version;
 	atomic_t active_ctr;
+	atomic_t barrier_cpu_count;
 	struct mutex mtx;
+	struct mutex barrier_mtx;
 	wait_queue_head_t wait_q;
+	struct completion barrier_completion;
 };
 
 #ifdef CONFIG_PRCU
@@ -48,6 +53,7 @@ void prcu_read_lock(void);
 void prcu_read_unlock(void);
 void synchronize_prcu(void);
 void call_prcu(struct rcu_head *head, rcu_callback_t func);
+void prcu_barrier(void);
 void prcu_init(void);
 void prcu_note_context_switch(void);
 int prcu_pending(void);
@@ -60,6 +66,7 @@ void prcu_check_callbacks(void);
 #define prcu_read_unlock() do {} while (0)
 #define synchronize_prcu() do {} while (0)
 #define call_prcu() do {} while (0)
+#define prcu_barrier() do {} while (0)
 #define prcu_init() do {} while (0)
 #define prcu_note_context_switch() do {} while (0)
 #define prcu_pending() 0
diff --git a/kernel/rcu/prcu.c b/kernel/rcu/prcu.c
index 373039c5..2664d091 100644
--- a/kernel/rcu/prcu.c
+++ b/kernel/rcu/prcu.c
@@ -15,6 +15,7 @@ struct prcu_struct global_prcu = {
 	.cb_version = ATOMIC64_INIT(0),
 	.active_ctr = ATOMIC_INIT(0),
 	.mtx = __MUTEX_INITIALIZER(global_prcu.mtx),
+	.barrier_mtx = __MUTEX_INITIALIZER(global_prcu.barrier_mtx),
 	.wait_q = __WAIT_QUEUE_HEAD_INITIALIZER(global_prcu.wait_q)
 };
 struct prcu_struct *prcu = &global_prcu;
@@ -250,6 +251,68 @@ static __latent_entropy void prcu_process_callbacks(struct softirq_action *unuse
 	local_irq_restore(flags);
 }
 
+/*
+ * PRCU callback function for prcu_barrier().
+ * If we are last, wake up the task executing prcu_barrier().
+ */
+static void prcu_barrier_callback(struct rcu_head *rhp)
+{
+	if (atomic_dec_and_test(&prcu->barrier_cpu_count))
+		complete(&prcu->barrier_completion);
+}
+
+/*
+ * Called with preemption disabled, and from cross-cpu IRQ context.
+ */
+static void prcu_barrier_func(void *info)
+{
+	struct prcu_local_struct *local = this_cpu_ptr(&prcu_local);
+
+	atomic_inc(&prcu->barrier_cpu_count);
+	call_prcu(&local->barrier_head, prcu_barrier_callback);
+}
+
+/* Waiting for all PRCU callbacks to complete. */
+void prcu_barrier(void)
+{
+	int cpu;
+
+	/* Take mutex to serialize concurrent prcu_barrier() requests. */
+	mutex_lock(&prcu->barrier_mtx);
+
+	/*
+	 * Initialize the count to one rather than to zero in order to
+	 * avoid a too-soon return to zero in case of a short grace period
+	 * (or preemption of this task).
+	 */
+	init_completion(&prcu->barrier_completion);
+	atomic_set(&prcu->barrier_cpu_count, 1);
+
+	/*
+	 * Register a new callback on each CPU using IPI to prevent races
+	 * with call_prcu(). When that callback is invoked, we will know
+	 * that all of the corresponding CPU's preceding callbacks have
+	 * been invoked.
+	 */
+	for_each_possible_cpu(cpu)
+		smp_call_function_single(cpu, prcu_barrier_func, NULL, 1);
+
+	/* Decrement the count as we initialize it to one. */
+	if (atomic_dec_and_test(&prcu->barrier_cpu_count))
+		complete(&prcu->barrier_completion);
+
+	/*
+	 * Now that we have an prcu_barrier_callback() callback on each
+	 * CPU, and thus each counted, remove the initial count.
+	 * Wait for all prcu_barrier_callback() callbacks to be invoked.
+	 */
+	wait_for_completion(&prcu->barrier_completion);
+
+	/* Other rcu_barrier() invocations can now safely proceed. */
+	mutex_unlock(&prcu->barrier_mtx);
+}
+EXPORT_SYMBOL(prcu_barrier);
+
 void prcu_init_local_struct(int cpu)
 {
 	struct prcu_local_struct *local;
-- 
2.14.1.729.g59c0ea183