Update barrier comments for membarrier/compiler barrier

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

diff --git a/src/rcu.h b/src/rcu.h
index d7b2de5faf62493aaf4316aab44d8487df491a31..42c3b1dac91a3a9325e69a88898186236a2d4a6f 100644 (file)
--- a/src/rcu.h
+++ b/src/rcu.h
@@ -55,14 +55,17 @@ unsigned int side_rcu_read_begin(struct side_rcu_gp_state *gp_state)
        (void) __atomic_add_fetch(&cpu_gp_state->count[period].begin, 1, __ATOMIC_RELAXED);
 fence:
        /*
-        * This memory barrier (A) ensures that the contents of the
-        * read-side critical section does not leak before the "begin"
-        * counter increment. It pairs with memory barriers (D) and (E).
+        * This compiler barrier (A) is paired with membarrier() at (C),
+        * (D), (E). It effectively upgrades this compiler barrier to a
+        * SEQ_CST fence with respect to the paired barriers.
         *
-        * This memory barrier (A) also ensures that the "begin"
-        * increment is before the "end" increment. It pairs with memory
-        * barrier (C). It is redundant with memory barrier (B) for that
-        * purpose.
+        * This barrier (A) ensures that the contents of the read-side
+        * critical section does not leak before the "begin" counter
+        * increment. It pairs with memory barriers (D) and (E).
+        *
+        * This barrier (A) also ensures that the "begin" increment is
+        * before the "end" increment. It pairs with memory barrier (C).
+        * It is redundant with barrier (B) for that purpose.
         */
        rseq_barrier();
        return period;
@@ -75,14 +78,17 @@ void side_rcu_read_end(struct side_rcu_gp_state *gp_state, unsigned int period)
        int cpu;
 
        /*
-        * This memory barrier (B) ensures that the contents of the
-        * read-side critical section does not leak after the "end"
-        * counter increment. It pairs with memory barriers (D) and (E).
+        * This compiler barrier (B) is paired with membarrier() at (C),
+        * (D), (E). It effectively upgrades this compiler barrier to a
+        * SEQ_CST fence with respect to the paired barriers.
+        *
+        * This barrier (B) ensures that the contents of the read-side
+        * critical section does not leak after the "end" counter
+        * increment. It pairs with memory barriers (D) and (E).
         *
-        * This memory barrier (B) also ensures that the "begin"
-        * increment is before the "end" increment. It pairs with memory
-        * barrier (C). It is redundant with memory barrier (A) for that
-        * purpose.
+        * This barrier (B) also ensures that the "begin" increment is
+        * before the "end" increment. It pairs with memory barrier (C).
+        * It is redundant with barrier (A) for that purpose.
         */
        rseq_barrier();