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SubjectRe: [PATCH 1/2] list: add hlist_unhashed_lockless()
On Thu, Nov 07, 2019 at 11:37:37AM -0800, Eric Dumazet wrote:
> 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: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>

I have queued this, but if you prefer it go some other way:

Acked-by: Paul E. McKenney <paulmck@kernel.org>

But shouldn't the uses in include/linux/rculist.h also be converted
into the patch below? If so, I will squash the following into your
patch.

Thanx, Paul

------------------------------------------------------------------------

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) \
\
 
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