Subject: Re: [PATCH] seq_file: add RCU versions of new hlist/list iterators

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 18 Feb 2010 17:12:47 -0800

On Thu, Feb 18, 2010 at 03:30:41PM -0800, Stephen Hemminger wrote:
> Many usages of seq_file use RCU protected lists, so non RCU
> iterators will not work safely.

Can't say that I am thrilled by the use of __list_for_each_rcu() and the
creation of __hlist_for_each(), but given the seq_list API, I don't see
a reasonable alternative.  So...

Acked-by: Paul E. McKenney <[email-masked]>

> Signed-off-by: Stephen Hemminger <[email-masked]>
> 
> ---
>  fs/seq_file.c            |   82 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/rculist.h  |    5 ++
>  include/linux/seq_file.h |   27 ++++++++++-----
>  3 files changed, 105 insertions(+), 9 deletions(-)
> 
> --- a/fs/seq_file.c	2010-02-18 15:08:53.228872265 -0800
> +++ b/fs/seq_file.c	2010-02-18 15:26:21.157402219 -0800
> @@ -695,6 +695,38 @@ struct list_head *seq_list_next(void *v,
>  }
>  EXPORT_SYMBOL(seq_list_next);
> 
> +struct list_head *seq_list_start_rcu(struct list_head *head, loff_t pos)
> +{
> +	struct list_head *lh;
> +
> +	__list_for_each_rcu(lh, head)
> +		if (pos-- == 0)
> +			return lh;
> +
> +	return NULL;
> +}
> +EXPORT_SYMBOL(seq_list_start_rcu);
> +
> +struct list_head *seq_list_start_head_rcu(struct list_head *head, loff_t pos)
> +{
> +	if (!pos)
> +		return head;
> +
> +	return seq_list_start_rcu(head, pos - 1);
> +}
> +EXPORT_SYMBOL(seq_list_start_head_rcu);
> +
> +struct list_head *seq_list_next_rcu(void *v, struct list_head *head,
> +				    loff_t *ppos)
> +{
> +	struct list_head *lh = v;
> +
> +	lh = rcu_dereference(lh->next);
> +	++*ppos;
> +	return lh == head ? NULL : lh;
> +}
> +EXPORT_SYMBOL(seq_list_next_rcu);
> +
>  /**
>   * seq_hlist_start - start an iteration of a hlist
>   * @head: the head of the hlist
> @@ -750,3 +782,53 @@ struct hlist_node *seq_hlist_next(void *
>  		return node->next;
>  }
>  EXPORT_SYMBOL(seq_hlist_next);
> +
> +/**
> + * seq_hlist_start_rcu - start an iteration of a hlist protected by RCU
> + * @head: the head of the hlist
> + * @pos:  the start position of the sequence
> + *
> + * Called at seq_file->op->start().
> + */
> +struct hlist_node *seq_hlist_start_rcu(struct hlist_head *head, loff_t pos)
> +{
> +	struct hlist_node *node;
> +
> +	__hlist_for_each_rcu(node, head)
> +		if (pos-- == 0)
> +			return node;
> +	return NULL;
> +}
> +EXPORT_SYMBOL(seq_hlist_start_rcu);
> +
> +/**
> + * seq_hlist_start_head_rcu - start an iteration of a hlist protected by RCU
> + * @head: the head of the hlist
> + * @pos:  the start position of the sequence
> + *
> + * Called at seq_file->op->start(). Call this function if you want to
> + * print a header at the top of the output.
> + */
> +struct hlist_node *seq_hlist_start_head_rcu(struct hlist_head *head, loff_t pos)
> +{
> +	if (!pos)
> +		return SEQ_START_TOKEN;
> +
> +	return seq_hlist_start_rcu(head, pos - 1);
> +}
> +EXPORT_SYMBOL(seq_hlist_start_rcu);
> +
> +/**
> + * seq_hlist_next_rcu - move to the next position of the hlist protected by RCU
> + * @v:    the current iterator
> + * @head: the head of the hlist
> + * @pos:  the current posision
> + *
> + * Called at seq_file->op->next().
> + */
> +struct hlist_node *seq_hlist_next_rcu(void *v, struct hlist_head *head,
> +				      loff_t *ppos)
> +{
> +	return rcu_dereference(seq_hlist_next(v, head, ppos));
> +}
> +EXPORT_SYMBOL(seq_hlist_next_rcu);
> --- a/include/linux/seq_file.h	2010-02-18 15:08:53.256872028 -0800
> +++ b/include/linux/seq_file.h	2010-02-18 15:09:14.128997914 -0800
> @@ -127,23 +127,32 @@ int seq_release_private(struct inode *, 
>  /*
>   * Helpers for iteration over list_head-s in seq_files
>   */
> -
>  extern struct list_head *seq_list_start(struct list_head *head,
> -		loff_t pos);
> +					loff_t pos);
>  extern struct list_head *seq_list_start_head(struct list_head *head,
> -		loff_t pos);
> +					     loff_t pos);
>  extern struct list_head *seq_list_next(void *v, struct list_head *head,
> -		loff_t *ppos);
> -
> +				       loff_t *ppos);
> +extern struct list_head *seq_list_start_rcu(struct list_head *head,
> +					    loff_t pos);
> +extern struct list_head *seq_list_start_head_rcu(struct list_head *head,
> +						 loff_t pos);
> +extern struct list_head *seq_list_next_rcu(void *v, struct list_head *head,
> +					   loff_t *ppos);
>  /*
>   * Helpers for iteration over hlist_head-s in seq_files
>   */
> -
>  extern struct hlist_node *seq_hlist_start(struct hlist_head *head,
> -		loff_t pos);
> +					  loff_t pos);
>  extern struct hlist_node *seq_hlist_start_head(struct hlist_head *head,
> -		loff_t pos);
> +					       loff_t pos);
>  extern struct hlist_node *seq_hlist_next(void *v, struct hlist_head *head,
> -		loff_t *ppos);
> +					 loff_t *ppos);
> 
> +extern struct hlist_node *seq_hlist_start_rcu(struct hlist_head *head,
> +					      loff_t pos);
> +extern struct hlist_node *seq_hlist_start_head_rcu(struct hlist_head *head,
> +						   loff_t pos);
> +extern struct hlist_node *seq_hlist_next_rcu(void *v, struct hlist_head *head,
> +					     loff_t *ppos);
>  #endif
> --- a/include/linux/rculist.h	2010-02-18 15:25:03.681496691 -0800
> +++ b/include/linux/rculist.h	2010-02-18 15:26:13.909286656 -0800
> @@ -406,6 +406,11 @@ static inline void hlist_add_after_rcu(s
>  		n->next->pprev = &n->next;
>  }
> 
> +#define __hlist_for_each(pos, head)			\
> +	for (pos = rcu_dereference((head)->first);	\
> +	     pos && ({ prefetch(pos->next); 1; });	\
> +	     pos = rcu_derference(pos->next))
> +
>  /**
>   * hlist_for_each_entry_rcu - iterate over rcu list of given type
>   * @tpos:	the type * to use as a loop cursor.