linux-kernel - Re: [PATCH v2] rcu: Add necessary WRITE

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Date:   Tue, 20 Jun 2023 15:26:44 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Huang <mmpgouride@...il.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rcu: Add necessary WRITE_ONCE()

On Tue, Jun 20, 2023 at 05:13:46PM +0000, Alan Huang wrote:
> Commit c54a2744497d("list: Add hlist_unhashed_lockless()") and
> commit 860c8802ace1("rcu: Use WRITE_ONCE() for assignments to
> ->pprev for hlist_nulls") added various WRITE_ONCE() to pair with
> the READ_ONCE() in hlist_unhashed_lockless(), but there are still
> some places where WRITE_ONCE() was not added, this commit adds that.
> 
> Also add WRITE_ONCE() to pair with the READ_ONCE() in hlist_empty().
> 
> Signed-off-by: Alan Huang <mmpgouride@...il.com>

On hlist_nulls_add_tail_rcu(), good catch, thank you!

On the others, are there really cases where a lockless read races with
the update?  At first glance, that sounds like a usage bug.  For example,
as I understand it, when you use something like hlist_del(), you are
supposed to ensure that there are no concurrent readers.  Which is the
point of the assignment of the special value LIST_POISON2, right?

Or is there some use case that I am missing?

If I am not missing something, then switching the non-RCU APIs to
WRITE_ONCE() would be a step backwards, because it would make it harder
for tools like KCSAN to find bugs.

							Thanx, Paul

> ---
> Changelog:
> V1 -> V2: 
>   Add WRITE_ONCE in hlist_del_init to pair with READ_ONCE in
>   hlist_unhashed_lockless.
> 
>  include/linux/list.h          | 9 +++++----
>  include/linux/list_nulls.h    | 2 +-
>  include/linux/rculist_nulls.h | 2 +-
>  3 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index ac366958ea..3a29b95bfe 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -912,7 +912,7 @@ static inline void hlist_del(struct hlist_node *n)
>  {
>  	__hlist_del(n);
>  	n->next = LIST_POISON1;
> -	n->pprev = LIST_POISON2;
> +	WRITE_ONCE(n->pprev, LIST_POISON2);
>  }
>  
>  /**
> @@ -925,7 +925,8 @@ static inline void hlist_del_init(struct hlist_node *n)
>  {
>  	if (!hlist_unhashed(n)) {
>  		__hlist_del(n);
> -		INIT_HLIST_NODE(n);
> +		n->next = NULL;
> +		WRITE_ONCE(n->pprev, NULL);
>  	}
>  }
>  
> @@ -1026,8 +1027,8 @@ static inline void hlist_move_list(struct hlist_head *old,
>  {
>  	new->first = old->first;
>  	if (new->first)
> -		new->first->pprev = &new->first;
> -	old->first = NULL;
> +		WRITE_ONCE(new->first->pprev, &new->first);
> +	WRITE_ONCE(old->first, NULL);
>  }
>  
>  #define hlist_entry(ptr, type, member) container_of(ptr,type,member)
> diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
> index fa6e8471bd..b63b0589fa 100644
> --- a/include/linux/list_nulls.h
> +++ b/include/linux/list_nulls.h
> @@ -95,7 +95,7 @@ static inline void hlist_nulls_add_head(struct hlist_nulls_node *n,
>  
>  	n->next = first;
>  	WRITE_ONCE(n->pprev, &h->first);
> -	h->first = n;
> +	WRITE_ONCE(h->first, n);
>  	if (!is_a_nulls(first))
>  		WRITE_ONCE(first->pprev, &n->next);
>  }
> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
> index ba4c00dd80..c65121655b 100644
> --- a/include/linux/rculist_nulls.h
> +++ b/include/linux/rculist_nulls.h
> @@ -138,7 +138,7 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
>  
>  	if (last) {
>  		n->next = last->next;
> -		n->pprev = &last->next;
> +		WRITE_ONCE(n->pprev, &last->next);
>  		rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
>  	} else {
>  		hlist_nulls_add_head_rcu(n, h);
> -- 
> 2.34.1
>