From 073dae2be2878ac1d8113930579c6459cd4acc92 Mon Sep 17 00:00:00 2001
From: Lukas Braun <lukas.braun@fau.de>
Date: Fri, 27 Jan 2017 21:13:03 +0100
Subject: [PATCH] comment on userlocks synchronization
---
goatherd.go | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/goatherd.go b/goatherd.go
index 29c9a2f..cc02fcc 100644
--- a/goatherd.go
+++ b/goatherd.go
@@ -259,14 +259,17 @@ func handle_conn(db *sql.DB, conn net.Conn) {
}
log.Printf("%v: %v", name, result)
- // name exists, get or create its lock
+ // name exists, so we can get or create its lock, double-checked locking style
+ // NOTE: the read lock is important to synchronize with the mutex initiali-
+ // zation, not just the map itself (i.e. a reader might observe a non-
+ // zeroed mutex if we used a concurrent map without a temporary variable
+ // and further memory barriers)
faildelay.RLock()
delay, exists := faildelay.userlocks[name]
faildelay.RUnlock()
if !exists {
debugf("[%v] not yet in faildelay.userlocks", remote)
- // no atomic upgrade with sync.RWMutex, so we have to do the lookup again
faildelay.Lock()
delay, exists = faildelay.userlocks[name]
if !exists {
--
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