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    SubjectRe: [PATCH] seq_file: add RCU versions of new hlist/list iterators
    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 <paulmck@linux.vnet.ibm.com>

    > Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
    >
    > ---
    > 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.


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