[Qemu-devel] [PATCH 3/3] rcu: detect missing rcu_register_thread()

[Paolo Bonzini <pbonzini@redhat.com>]

Use an "impossible" value for the .depth field in order to quickly
detect threads that have not registered themselves with the RCU
subsystem.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 include/qemu/rcu.h |  4 +++-
 util/rcu.c         | 18 ++++++++++++++++--
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
index 7df1e86..4facb35 100644
--- a/include/qemu/rcu.h
+++ b/include/qemu/rcu.h
@@ -82,7 +82,9 @@ static inline void rcu_read_lock(void)
     struct rcu_reader_data *p_rcu_reader = &rcu_reader;
     unsigned ctr;
 
-    if (p_rcu_reader->depth++ > 0) {
+    p_rcu_reader->depth++;
+    assert(p_rcu_reader->depth >= 1);
+    if (p_rcu_reader->depth > 1) {
         return;
     }
 
diff --git a/util/rcu.c b/util/rcu.c
index 8830295..435cdbe 100644
--- a/util/rcu.c
+++ b/util/rcu.c
@@ -63,8 +63,11 @@ static inline int rcu_gp_ongoing(unsigned long *ctr)
 
 /* Written to only by each individual reader. Read by both the reader and the
  * writers.
+ *
+ * Initializing the depth to -1 causes an assertion failure on the first
+ * call to rcu_read_lock() if the thread does not call rcu_register_thread().
  */
-__thread struct rcu_reader_data rcu_reader;
+__thread struct rcu_reader_data rcu_reader = { .depth = -1 };
 
 /* Protected by rcu_gp_lock.  */
 typedef QLIST_HEAD(, rcu_reader_data) ThreadList;
@@ -277,7 +280,12 @@ static void rcu_unregister_thread_notify(Notifier *n, void *data)
 
 void rcu_register_thread(void)
 {
-    assert(rcu_reader.ctr == 0);
+    /* rcu_reader.depth is also used to detect whether the thread is
+     * registered.
+     */
+    assert(rcu_reader.depth == -1);
+    rcu_reader.depth = 0;
+
     qemu_mutex_lock(&rcu_gp_lock);
     QLIST_INSERT_HEAD(&registry, &rcu_reader, node);
     qemu_mutex_unlock(&rcu_gp_lock);
@@ -288,6 +296,12 @@ void rcu_register_thread(void)
 
 void rcu_unregister_thread(void)
 {
+    /* Resetting the depth to -1 causes an assertion failure on the next
+     * call to rcu_read_lock().
+     */
+    assert(rcu_reader.depth == 0);
+    rcu_reader.depth = -1;
+
     qemu_mutex_lock(&rcu_gp_lock);
     QLIST_REMOVE(&rcu_reader, node);
     qemu_mutex_unlock(&rcu_gp_lock);
-- 
2.4.3