rcu: remove qatomic_mb_set, expand comments

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/rcu.h

@@ -87,7 +87,10 @@ static inline void rcu_read_lock(void)
     ctr = qatomic_read(&rcu_gp_ctr);
     qatomic_set(&p_rcu_reader->ctr, ctr);
 
-    /* Write p_rcu_reader->ctr before reading RCU-protected pointers.  */
+    /*
+     * Read rcu_gp_ptr and write p_rcu_reader->ctr before reading
+     * RCU-protected pointers.
+     */
     smp_mb_placeholder();
 }
 

util/rcu.c

@@ -83,12 +83,6 @@ static void wait_for_readers(void)
          */
         qemu_event_reset(&rcu_gp_event);
 
-        /* Instead of using qatomic_mb_set for index->waiting, and
-         * qatomic_mb_read for index->ctr, memory barriers are placed
-         * manually since writes to different threads are independent.
-         * qemu_event_reset has acquire semantics, so no memory barrier
-         * is needed here.
-         */
         QLIST_FOREACH(index, &registry, node) {
             qatomic_set(&index->waiting, true);
         }
@@ -96,6 +90,10 @@ static void wait_for_readers(void)
         /* Here, order the stores to index->waiting before the loads of
          * index->ctr.  Pairs with smp_mb_placeholder() in rcu_read_unlock(),
          * ensuring that the loads of index->ctr are sequentially consistent.
+         *
+         * If this is the last iteration, this barrier also prevents
+         * frees from seeping upwards, and orders the two wait phases
+         * on architectures with 32-bit longs; see synchronize_rcu().
          */
         smp_mb_global();
 
@@ -104,7 +102,7 @@ static void wait_for_readers(void)
                 QLIST_REMOVE(index, node);
                 QLIST_INSERT_HEAD(&qsreaders, index, node);
 
-                /* No need for mb_set here, worst of all we
+                /* No need for memory barriers here, worst of all we
                  * get some extra futex wakeups.
                  */
                 qatomic_set(&index->waiting, false);
@@ -149,26 +147,26 @@ void synchronize_rcu(void)
 
     /* Write RCU-protected pointers before reading p_rcu_reader->ctr.
      * Pairs with smp_mb_placeholder() in rcu_read_lock().
+     *
+     * Also orders write to RCU-protected pointers before
+     * write to rcu_gp_ctr.
      */
     smp_mb_global();
 
     QEMU_LOCK_GUARD(&rcu_registry_lock);
     if (!QLIST_EMPTY(&registry)) {
-        /* In either case, the qatomic_mb_set below blocks stores that free
-         * old RCU-protected pointers.
-         */
         if (sizeof(rcu_gp_ctr) < 8) {
             /* For architectures with 32-bit longs, a two-subphases algorithm
              * ensures we do not encounter overflow bugs.
              *
              * Switch parity: 0 -> 1, 1 -> 0.
              */
-            qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
+            qatomic_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
             wait_for_readers();
-            qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
+            qatomic_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
         } else {
             /* Increment current grace period.  */
-            qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
+            qatomic_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
         }
 
         wait_for_readers();