From a25e073a2b526824fe9c4631e5781a744bcc3369 Mon Sep 17 00:00:00 2001
From: Samy Al Bahra <sbahra@backtrace.io>
Date: Wed, 15 Mar 2017 00:52:51 -0400
Subject: [PATCH] ck_epoch: allow record sharing and reentrancy for write-side
 operations.

---
 include/ck_epoch.h | 29 ++++++++++++++++++++++++++++-
 src/ck_epoch.c     | 17 ++++++++++-------
 2 files changed, 38 insertions(+), 8 deletions(-)

diff --git a/include/ck_epoch.h b/include/ck_epoch.h
index e7ce5bc..21bb52c 100644
--- a/include/ck_epoch.h
+++ b/include/ck_epoch.h
@@ -92,7 +92,8 @@ struct ck_epoch_record {
 	} local CK_CC_CACHELINE;
 	unsigned int n_pending;
 	unsigned int n_peak;
-	unsigned long n_dispatch;
+	unsigned int n_dispatch;
+	unsigned int unused;
 	ck_stack_t pending[CK_EPOCH_LENGTH];
 	ck_stack_entry_t record_next;
 } CK_CC_CACHELINE;
@@ -179,7 +180,12 @@ ck_epoch_end(ck_epoch_record_t *record, ck_epoch_section_t *section)
  * Defers the execution of the function pointed to by the "cb"
  * argument until an epoch counter loop. This allows for a
  * non-blocking deferral.
+ *
+ * We can get away without a fence here due to the monotonic nature
+ * of the epoch counter. Worst case, this will result in some delays
+ * before object destruction.
  */
+/*
 CK_CC_FORCE_INLINE static void
 ck_epoch_call(ck_epoch_record_t *record,
 	      ck_epoch_entry_t *entry,
@@ -194,6 +200,27 @@ ck_epoch_call(ck_epoch_record_t *record,
 	ck_stack_push_spnc(&record->pending[offset], &entry->stack_entry);
 	return;
 }
+*/
+
+/*
+ * Same as ck_epoch_call, but allows for records to be shared and is reentrant.
+ */
+CK_CC_FORCE_INLINE static void
+ck_epoch_call(ck_epoch_record_t *record,
+	      ck_epoch_entry_t *entry,
+	      ck_epoch_cb_t *function)
+{
+	struct ck_epoch *epoch = record->global;
+	unsigned int e = ck_pr_load_uint(&epoch->epoch);
+	unsigned int offset = e & (CK_EPOCH_LENGTH - 1);
+
+	ck_pr_inc_uint(&record->n_pending);
+	entry->function = function;
+
+	/* Store fence is implied by push operation. */
+	ck_stack_push_upmc(&record->pending[offset], &entry->stack_entry);
+	return;
+}
 
 void ck_epoch_init(ck_epoch_t *);
 ck_epoch_record_t *ck_epoch_recycle(ck_epoch_t *);
diff --git a/src/ck_epoch.c b/src/ck_epoch.c
index 69ba6e0..018011c 100644
--- a/src/ck_epoch.c
+++ b/src/ck_epoch.c
@@ -345,11 +345,10 @@ ck_epoch_dispatch(struct ck_epoch_record *record, unsigned int e)
 {
 	unsigned int epoch = e & (CK_EPOCH_LENGTH - 1);
 	ck_stack_entry_t *head, *next, *cursor;
+	unsigned int n_pending, n_peak;
 	unsigned int i = 0;
 
-	head = CK_STACK_FIRST(&record->pending[epoch]);
-	ck_stack_init(&record->pending[epoch]);
-
+	head = ck_stack_batch_pop_upmc(&record->pending[epoch]);
 	for (cursor = head; cursor != NULL; cursor = next) {
 		struct ck_epoch_entry *entry =
 		    ck_epoch_entry_container(cursor);
@@ -359,11 +358,15 @@ ck_epoch_dispatch(struct ck_epoch_record *record, unsigned int e)
 		i++;
 	}
 
-	if (record->n_pending > record->n_peak)
-		record->n_peak = record->n_pending;
+	n_peak = ck_pr_load_uint(&record->n_peak);
+	n_pending = ck_pr_load_uint(&record->n_pending);
+
+	/* We don't require accuracy around peak calculation. */
+	if (n_pending > n_peak)
+		ck_pr_store_uint(&record->n_peak, n_peak);
 
-	record->n_dispatch += i;
-	record->n_pending -= i;
+	ck_pr_add_uint(&record->n_dispatch, i);
+	ck_pr_sub_uint(&record->n_pending, i);
 	return;
 }