srcu: Document srcu_flip() memory-barrier D relation to SRCU-fast

The smp_mb() memory barrier at the end of srcu_flip() has a comment,
but that comment does not make it clear that this memory barrier is an
optimization, as opposed to being needed for correctness.  This commit
therefore adds this information and points out that it is omitted
for SRCU-fast, where a much heavier weight synchronize_srcu() would
be required.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: <bpf@vger.kernel.org>

Author: paulmckrcu

kernel/rcu/srcutree.c

@@ -1168,6 +1168,16 @@ static void srcu_flip(struct srcu_struct *ssp)
 	 * counter update.  Note that both this memory barrier and the
 	 * one in srcu_readers_active_idx_check() provide the guarantee
 	 * for __srcu_read_lock().
+	 *
+	 * Note that this is a performance optimization, in which we spend
+	 * an otherwise unnecessary smp_mb() in order to reduce the number
+	 * of full per-CPU-variable scans in srcu_readers_lock_idx() and
+	 * srcu_readers_unlock_idx().  But this performance optimization
+	 * is not so optimal for SRCU-fast, where we would be spending
+	 * not smp_mb(), but rather synchronize_rcu().  At the same time,
+	 * the overhead of the smp_mb() is in the noise, so there is no
+	 * point in omitting it in the SRCU-fast case.  So the same code
+	 * is executed either way.
 	 */
 	smp_mb(); /* D */  /* Pairs with C. */
 }