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list: Add hlist_unhashed_lockless()

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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>
This commit is contained in:
Eric Dumazet
2019-11-07 11:37:37 -08:00
committed by Paul E. McKenney
parent f452ee096d
commit c54a274449
2 changed files with 33 additions and 23 deletions
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32
include/linux/list.h
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@@ -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 */