urcu: Add extra "engineering safety factor" memory barrier in update_counter_and_wait()

[urcu.git] / urcu-qsbr.c

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index 84789f2ebfdd4fa89def6fb56274595739a51d02..c678d38779eb683f1f78872a87e853a4c281f43d 100644 (file)
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -120,6 +120,17 @@ static void update_counter_and_wait(void)
        STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 #endif /* !(BITS_PER_LONG < 64) */
 
+       /*
+        * Enforce compiler-order of store to rcu_gp_ctr before before
+        * load rcu_reader ctr.
+        * This ensures synchronize_rcu() cannot be starved by readers.
+        *
+        * Adding a smp_mb() which is _not_ formally required, but makes the
+        * model easier to understand. It does not have a big performance impact
+        * anyway, given this is the write-side.
+        */
+       smp_mb();
+
        /*
         * Wait for each thread rcu_reader_qs_gp count to become 0.
         */