Update urcu and urcu-qsbr, make compiler barriers explicit

[urcu.git] / urcu-qsbr.c

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index 155505abc7843385fafef29de57509ceb096519c..b043b8592e7f04f1606bd3b734b2130e09c260e9 100644 (file)
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -113,13 +113,28 @@ static void update_counter_and_wait(void)
        struct rcu_reader *index, *tmp;
 
 #if (BITS_PER_LONG < 64)
-       /* Switch parity: 1 -> 0, 0 -> 1 */
+       /* Switch parity: 0 -> 1, 1 -> 0 */
        STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 #else  /* !(BITS_PER_LONG < 64) */
        /* Increment current G.P. */
        STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 #endif /* !(BITS_PER_LONG < 64) */
 
+       /*
+        * Must commit rcu_gp_ctr update to memory before waiting for quiescent
+        * state. Failure to do so could result in the writer waiting forever
+        * while new readers are always accessing data (no progress). Enforce
+        * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
+        */
+       barrier();
+
+       /*
+        * 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.
         */
@@ -197,11 +212,12 @@ void synchronize_rcu(void)
 
        /*
         * Must finish waiting for quiescent state for parity 0 before
-        * committing qparity update to memory. Failure to do so could result in
-        * the writer waiting forever while new readers are always accessing
-        * data (no progress).
-        * Ensured by STORE_SHARED and LOAD_SHARED.
+        * committing next rcu_gp_ctr update to memory. Failure to do so could
+        * result in the writer waiting forever while new readers are always
+        * accessing data (no progress).  Enforce compiler-order of load
+        * rcu_reader ctr before store to rcu_gp_ctr.
         */
+       barrier();
 
        /*
         * Adding a smp_mb() which is _not_ formally required, but makes the