[lttng-dev] [PATCH latency-tracker] Fix: use local ops for freelist per-cpu counter

[mathieu.desnoyers@efficios.com (Mathieu Desnoyers)]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
---
 tracker_private.h     |  3 ++-
 wrapper/freelist-ll.h | 21 +++++++++++++++------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/tracker_private.h b/tracker_private.h
index 03386b8..fc27f80 100644
--- a/tracker_private.h
+++ b/tracker_private.h
@@ -7,6 +7,7 @@
 
 #include <linux/workqueue.h>
 #include <linux/irq_work.h>
+#include <asm/local.h>
 
 //#include "wrapper/ht.h"
 //#include "rculfhash-internal.h"
@@ -18,7 +19,7 @@ struct numa_pool {
 };
 
 struct per_cpu_ll {
-	int current_count;
+	local_t current_count;
 	struct numa_pool *pool;
 	struct llist_head llist;
 };
diff --git a/wrapper/freelist-ll.h b/wrapper/freelist-ll.h
index 5e4d18e..44db722 100644
--- a/wrapper/freelist-ll.h
+++ b/wrapper/freelist-ll.h
@@ -242,8 +242,8 @@ int free_per_cpu_llist(struct latency_tracker *tracker)
 		if (!list)
 			continue;
 		cnt = free_event_list(list);
-		printk("freed %d on cpu %d (%d)\n", cnt, cpu,
-				ll->current_count);
+		printk("freed %d on cpu %d (%ld)\n", cnt, cpu,
+				local_read(&ll->current_count));
 		total_cnt += cnt;
 	}
 
@@ -284,10 +284,14 @@ struct llist_node *per_cpu_get(struct latency_tracker *tracker)
 	struct per_cpu_ll *ll;
 
 	ll = lttng_this_cpu_ptr(tracker->per_cpu_ll);
+	/*
+	 * Decrement the current count after we successfully remove an
+	 * element from the list.
+	 */
 	node = llist_del_first(&ll->llist);
 	if (node) {
-		ll->current_count--;
-		WARN_ON_ONCE(ll->current_count < 0);
+		local_dec(&ll->current_count);
+		WARN_ON_ONCE(local_read(&ll->current_count) < 0);
 		return node;
 	}
 	return llist_del_first(&ll->pool->llist);
@@ -336,12 +340,17 @@ void __wrapper_freelist_put_event(struct latency_tracker *tracker,
 	if (e->pool != ll->pool) {
 //		printk("DEBUG cross-pool put_event\n");
 		llist_add(&e->llist, &ll->pool->llist);
-	} else if (ll->current_count < FREELIST_PERCPU_CACHE) {
+	} else if (local_read(&ll->current_count) < FREELIST_PERCPU_CACHE) {
 		/*
 		 * Fill our local cache if needed.
+		 * We need to increment current_count before we add the
+		 * element to the list, because when we successfully
+		 * remove an element from the list, we expect that the
+		 * counter is never negative. An interrupt can observe
+		 * the intermediate state.
 		 */
+		local_inc(&ll->current_count);
 		llist_add(&e->llist, &ll->llist);
-		ll->current_count++;
 	} else {
 		/*
 		 * Add to our NUMA pool.
-- 
2.1.4