Document QSBR interaction with mutexes

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/README b/README
index 1e76f2d1b098bb9579d573fcf8f36211c3c7c5f9..e3800faad2d10cc5789c0a622c14f5ade9962ae1 100644 (file)
--- a/README
+++ b/README
@@ -187,6 +187,15 @@ Interaction with mutexes
        mutex in its dependency chain) should not be acquired from within a RCU
        read-side critical section.
 
+       This is especially important to understand in the context of the
+       QSBR flavor: a registered reader thread being "online" by
+       default should be considered as within a RCU read-side critical
+       section unless explicitly put "offline". Therefore, if
+       synchronize_rcu() is called with a mutex held, this mutex, as
+       well as any mutex which has this mutex in its dependency chain
+       should only be taken when the RCU reader thread is "offline"
+       (this can be performed by calling rcu_thread_offline()).
+
 Usage of DEBUG_RCU
 
        DEBUG_RCU is used to add internal debugging self-checks to the