[RFC PATCH 01/21] list: Remove hlist_unhashed_lockless()

[Will Deacon]

Commit c54a2744497d ("list: Add hlist_unhashed_lockless()") added a
"lockless" variant of hlist_unhashed() that uses READ_ONCE() to access
the 'pprev' pointer of the 'hlist_node', the intention being that this
could be used by 'timer_pending()' to silence a KCSAN warning. As well
as forgetting to add the caller, the patch also sprinkles
{READ,WRITE}_ONCE() invocations over the standard (i.e. non-RCU) hlist
code, which is undesirable for a number of reasons:

  1. It gives the misleading impression that the non-RCU hlist code is
     in some way lock-free (despite the notable absence of any memory
     barriers!) and silences KCSAN in such cases.

  2. It unnecessarily penalises code generation for non-RCU hlist users

  3. It makes it difficult to introduce list integrity checks because
     of the possibility of concurrent callers.

Retain the {READ,WRITE}_ONCE() invocations for the RCU hlist code, but
remove them from the non-RCU implementation. Remove the unused
'hlist_unhashed_lockless()' function entirely and add the READ_ONCE()
to hlist_unhashed(), as we do already for hlist_empty() already.

Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 include/linux/list.h | 33 ++++++++++-----------------------
 1 file changed, 10 insertions(+), 23 deletions(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index 884216db3246..4fed5a0f9b77 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -777,19 +777,6 @@ static inline void INIT_HLIST_NODE(struct hlist_node *h)
  * node in unhashed state, but hlist_nulls_del() does not.
  */
 static inline int hlist_unhashed(const struct hlist_node *h)
-{
-	return !h->pprev;
-}
-
-/**
- * hlist_unhashed_lockless - Version of hlist_unhashed for lockless use
- * @h: Node to be checked
- *
- * This variant of hlist_unhashed() must be used in lockless contexts
- * to avoid potential load-tearing.  The READ_ONCE() is paired with the
- * various WRITE_ONCE() in hlist helpers that are defined below.
- */
-static inline int hlist_unhashed_lockless(const struct hlist_node *h)
 {
 	return !READ_ONCE(h->pprev);
 }
@@ -852,11 +839,11 @@ static inline void hlist_del_init(struct hlist_node *n)
 static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
 {
 	struct hlist_node *first = h->first;
-	WRITE_ONCE(n->next, first);
+	n->next = first;
 	if (first)
-		WRITE_ONCE(first->pprev, &n->next);
+		first->pprev = &n->next;
 	WRITE_ONCE(h->first, n);
-	WRITE_ONCE(n->pprev, &h->first);
+	n->pprev = &h->first;
 }
 
 /**
@@ -867,9 +854,9 @@ static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
 static inline void hlist_add_before(struct hlist_node *n,
 				    struct hlist_node *next)
 {
-	WRITE_ONCE(n->pprev, next->pprev);
-	WRITE_ONCE(n->next, next);
-	WRITE_ONCE(next->pprev, &n->next);
+	n->pprev = next->pprev;
+	n->next = next;
+	next->pprev = &n->next;
 	WRITE_ONCE(*(n->pprev), n);
 }
 
@@ -881,12 +868,12 @@ static inline void hlist_add_before(struct hlist_node *n,
 static inline void hlist_add_behind(struct hlist_node *n,
 				    struct hlist_node *prev)
 {
-	WRITE_ONCE(n->next, prev->next);
-	WRITE_ONCE(prev->next, n);
-	WRITE_ONCE(n->pprev, &prev->next);
+	n->next = prev->next;
+	prev->next = n;
+	n->pprev = &prev->next;
 
 	if (n->next)
-		WRITE_ONCE(n->next->pprev, &n->next);
+		n->next->pprev  = &n->next;
 }
 
 /**
-- 
2.20.1