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Subject[PATCH tip/core/rcu 2/9] list: Add hlist_unhashed_lockless()
Date
From: Eric Dumazet <edumazet@google.com>

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>
---
include/linux/list.h | 32 +++++++++++++++++++++-----------
include/linux/rculist.h | 24 ++++++++++++------------
2 files changed, 33 insertions(+), 23 deletions(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index 85c9255..61f5aaf 100644
--- 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 61c6728a..4b7ae1b 100644
--- 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) \
--
2.9.5
\
 
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