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_plugin.h b/kernel/rcu/tree_plugin.h
index 6abb03d..b023e54 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -779,8 +779,10 @@
}
if (rnp->parent == NULL) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
- if (wake)
+ if (wake) {
+ smp_mb(); /* EGP done before wake_up(). */
wake_up(&sync_rcu_preempt_exp_wq);
+ }
break;
}
mask = rnp->grpmask;
@@ -1852,6 +1854,7 @@
/* Wait for callbacks from earlier instance to complete. */
wait_event(oom_callback_wq, atomic_read(&oom_callback_count) == 0);
+ smp_mb(); /* Ensure callback reuse happens after callback invocation. */
/*
* Prevent premature wakeup: ensure that all increments happen
@@ -2250,6 +2253,7 @@
trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
TPS("Sleep"));
wait_event_interruptible(rdp->nocb_wq, rdp->nocb_head);
+ /* Memory barrier provide by xchg() below. */
} else if (firsttime) {
firsttime = 0;
trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,