[PATCH 4.13 006/109] srcu: Provide ordering for CPU not involved in grace period

[Greg Kroah-Hartman]

4.13-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

commit 35732cf9dd38b1efb0f2f22c91c61b51337d1ac3 upstream.

Tree RCU guarantees that every online CPU has a memory barrier between
any given grace period and any of that CPU's RCU read-side sections that
must be ordered against that grace period.  Since RCU doesn't always
know where read-side critical sections are, the actual implementation
guarantees order against prior and subsequent non-idle non-offline code,
whether in an RCU read-side critical section or not.  As a result, there
does not need to be a memory barrier at the end of synchronize_rcu()
and friends because the ordering internal to the grace period has
ordered every CPU's post-grace-period execution against each CPU's
pre-grace-period execution, again for all non-idle online CPUs.

In contrast, SRCU can have non-idle online CPUs that are completely
uninvolved in a given SRCU grace period, for example, a CPU that
never runs any SRCU read-side critical sections and took no part in
the grace-period processing.  It is in theory possible for a given
synchronize_srcu()'s wakeup to be delivered to a CPU that was completely
uninvolved in the prior SRCU grace period, which could mean that the
code following that synchronize_srcu() would end up being unordered with
respect to both the grace period and any pre-existing SRCU read-side
critical sections.

This commit therefore adds an smp_mb() to the end of __synchronize_srcu(),
which prevents this scenario from occurring.

Reported-by: Lance Roy <ldr709@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Acked-by: Lance Roy <ldr709@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/rcu/srcutree.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -896,6 +896,15 @@ static void __synchronize_srcu(struct sr
 	__call_srcu(sp, &rcu.head, wakeme_after_rcu, do_norm);
 	wait_for_completion(&rcu.completion);
 	destroy_rcu_head_on_stack(&rcu.head);
+
+	/*
+	 * Make sure that later code is ordered after the SRCU grace
+	 * period.  This pairs with the raw_spin_lock_irq_rcu_node()
+	 * in srcu_invoke_callbacks().  Unlike Tree RCU, this is needed
+	 * because the current CPU might have been totally uninvolved with
+	 * (and thus unordered against) that grace period.
+	 */
+	smp_mb();
 }
 
 /**