rcu: Add READ_ONCE() to rcu_segcblist ->tails[]

paulmckrcu · paulmckrcu · commit bfeebe24212d · 2020-02-20T15:58:22.000-08:00
The rcu_segcblist structure's -&gt;tails[] array entries are read
locklessly, so this commit adds the READ_ONCE() to a load in order to
avoid destructive compiler optimizations.

This data race was reported by KCSAN.

Signed-off-by: Paul E. McKenney &lt;paulmck@kernel.org&gt;
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
@@ -182,7 +182,7 @@ void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
 bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp)
 {
 	return rcu_segcblist_is_enabled(rsclp) &&
-	       &rsclp->head != rsclp->tails[RCU_DONE_TAIL];
+	       &rsclp->head != READ_ONCE(rsclp->tails[RCU_DONE_TAIL]);
 }
 
 /*

Original file line number	Diff line number	Diff line change
`@@ -182,7 +182,7 @@ void rcu_segcblist_offload(struct rcu_segcblist *rsclp)`
`182`	`182`	`bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp)`
`183`	`183`	`{`
`184`	`184`	`return rcu_segcblist_is_enabled(rsclp) &&`
`185`		`- &rsclp->head != rsclp->tails[RCU_DONE_TAIL];`
	`185`	`+ &rsclp->head != READ_ONCE(rsclp->tails[RCU_DONE_TAIL]);`
`186`	`186`	`}`
`187`	`187`
`188`	`188`	`/*`