list: Add hlist_unhashed_lockless()

We would like to use hlist_unhashed() from timer_pending(),
which runs without protection of a lock.

Note that other callers might also want to use this variant.

Instead of forcing a READ_ONCE() for all hlist_unhashed()
callers, add a new helper with an explicit _lockless suffix
in the name to better document what is going on.

Also add various WRITE_ONCE() in __hlist_del(), hlist_add_head()
and hlist_add_before()/hlist_add_behind() to pair with
the READ_ONCE().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
[ paulmck: Also add WRITE_ONCE() to rculist.h. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/include/linux/list.h b/include/linux/list.h
index 85c9255..61f5aaf 100644 (file)
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -749,6 +749,16 @@ static inline int hlist_unhashed(const struct hlist_node *h)
        return !h->pprev;
 }
 
+/* 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);
+}
+
 static inline int hlist_empty(const struct hlist_head *h)
 {
        return !READ_ONCE(h->first);
@@ -761,7 +771,7 @@ static inline void __hlist_del(struct hlist_node *n)
 
        WRITE_ONCE(*pprev, next);
        if (next)
-               next->pprev = pprev;
+               WRITE_ONCE(next->pprev, pprev);
 }
 
 static inline void hlist_del(struct hlist_node *n)
@@ -782,32 +792,32 @@ 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;
-       n->next = first;
+       WRITE_ONCE(n->next, first);
        if (first)
-               first->pprev = &n->next;
+               WRITE_ONCE(first->pprev, &n->next);
        WRITE_ONCE(h->first, n);
-       n->pprev = &h->first;
+       WRITE_ONCE(n->pprev, &h->first);
 }
 
 /* next must be != NULL */
 static inline void hlist_add_before(struct hlist_node *n,
                                        struct hlist_node *next)
 {
-       n->pprev = next->pprev;
-       n->next = next;
-       next->pprev = &n->next;
+       WRITE_ONCE(n->pprev, next->pprev);
+       WRITE_ONCE(n->next, next);
+       WRITE_ONCE(next->pprev, &n->next);
        WRITE_ONCE(*(n->pprev), n);
 }
 
 static inline void hlist_add_behind(struct hlist_node *n,
                                    struct hlist_node *prev)
 {
-       n->next = prev->next;
-       prev->next = n;
-       n->pprev = &prev->next;
+       WRITE_ONCE(n->next, prev->next);
+       WRITE_ONCE(prev->next, n);
+       WRITE_ONCE(n->pprev, &prev->next);
 
        if (n->next)
-               n->next->pprev  = &n->next;
+               WRITE_ONCE(n->next->pprev, &n->next);
 }
 
 /* after that we'll appear to be on some hlist and hlist_del will work */

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 61c6728..4b7ae1b 100644 (file)
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -173,7 +173,7 @@ static inline void hlist_del_init_rcu(struct hlist_node *n)
 {
        if (!hlist_unhashed(n)) {
                __hlist_del(n);
-               n->pprev = NULL;
+               WRITE_ONCE(n->pprev, NULL);
        }
 }
 
@@ -473,7 +473,7 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
 static inline void hlist_del_rcu(struct hlist_node *n)
 {
        __hlist_del(n);
-       n->pprev = LIST_POISON2;
+       WRITE_ONCE(n->pprev, LIST_POISON2);
 }
 
 /**
@@ -489,11 +489,11 @@ static inline void hlist_replace_rcu(struct hlist_node *old,
        struct hlist_node *next = old->next;
 
        new->next = next;
-       new->pprev = old->pprev;
+       WRITE_ONCE(new->pprev, old->pprev);
        rcu_assign_pointer(*(struct hlist_node __rcu **)new->pprev, new);
        if (next)
-               new->next->pprev = &new->next;
-       old->pprev = LIST_POISON2;
+               WRITE_ONCE(new->next->pprev, &new->next);
+       WRITE_ONCE(old->pprev, LIST_POISON2);
 }
 
 /*
@@ -528,10 +528,10 @@ static inline void hlist_add_head_rcu(struct hlist_node *n,
        struct hlist_node *first = h->first;
 
        n->next = first;
-       n->pprev = &h->first;
+       WRITE_ONCE(n->pprev, &h->first);
        rcu_assign_pointer(hlist_first_rcu(h), n);
        if (first)
-               first->pprev = &n->next;
+               WRITE_ONCE(first->pprev, &n->next);
 }
 
 /**
@@ -564,7 +564,7 @@ static inline void hlist_add_tail_rcu(struct hlist_node *n,
 
        if (last) {
                n->next = last->next;
-               n->pprev = &last->next;
+               WRITE_ONCE(n->pprev, &last->next);
                rcu_assign_pointer(hlist_next_rcu(last), n);
        } else {
                hlist_add_head_rcu(n, h);
@@ -592,10 +592,10 @@ static inline void hlist_add_tail_rcu(struct hlist_node *n,
 static inline void hlist_add_before_rcu(struct hlist_node *n,
                                        struct hlist_node *next)
 {
-       n->pprev = next->pprev;
+       WRITE_ONCE(n->pprev, next->pprev);
        n->next = next;
        rcu_assign_pointer(hlist_pprev_rcu(n), n);
-       next->pprev = &n->next;
+       WRITE_ONCE(next->pprev, &n->next);
 }
 
 /**
@@ -620,10 +620,10 @@ static inline void hlist_add_behind_rcu(struct hlist_node *n,
                                        struct hlist_node *prev)
 {
        n->next = prev->next;
-       n->pprev = &prev->next;
+       WRITE_ONCE(n->pprev, &prev->next);
        rcu_assign_pointer(hlist_next_rcu(prev), n);
        if (n->next)
-               n->next->pprev = &n->next;
+               WRITE_ONCE(n->next->pprev, &n->next);
 }
 
 #define __hlist_for_each_rcu(pos, head)                                \