rcu: Fix and comment ordering around wait_event()

It is all too easy to forget that wait_event() does not necessarily
imply a full memory barrier.  The case where it does not is where the
condition transitions to true just as wait_event() starts execution.
This is actually a feature: The standard use of wait_event() involves
locking, in which case the locks provide the needed ordering (you hold a
lock across the wake_up() and acquire that same lock after wait_event()
returns).

Given that I did forget that wait_event() does not necessarily imply a
full memory barrier in one case, this commit fixes that case.  This commit
also adds comments calling out the placement of existing memory barriers
relied on by wait_event() calls.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5243ebea0fc1..abef9c358d47 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1533,6 +1533,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
                 rdp = this_cpu_ptr(rsp->rda);
                 if (rnp == rdp->mynode)
                         __note_gp_changes(rsp, rnp, rdp);
+                /* smp_mb() provided by prior unlock-lock pair. */
                 nocb += rcu_future_gp_cleanup(rsp, rnp);
                 raw_spin_unlock_irq(&rnp->lock);
                 cond_resched();
@@ -1577,6 +1578,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                         wait_event_interruptible(rsp->gp_wq,
                                                  ACCESS_ONCE(rsp->gp_flags) &
                                                  RCU_GP_FLAG_INIT);
+                        /* Locking provides needed memory barrier. */
                         if (rcu_gp_init(rsp))
                                 break;
                         cond_resched();
@@ -1606,6 +1608,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                                         (!ACCESS_ONCE(rnp->qsmask) &&
                                          !rcu_preempt_blocked_readers_cgp(rnp)),
                                         j);
+                        /* Locking provides needed memory barriers. */
                         /* If grace period done, leave loop. */
                         if (!ACCESS_ONCE(rnp->qsmask) &&
                             !rcu_preempt_blocked_readers_cgp(rnp))