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>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5243ebe..abef9c3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1533,6 +1533,7 @@
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 @@
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 @@
(!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))