[RFC patch 18/41] Seq_file add support for sorted list

[Mathieu Desnoyers]

Add support for sorted list in seq_file. It aims at changing the way
/proc/modules and kallsyms iterates on the module list to remove a race between
module unload and module/symbol listing.

The list is sorted by ascending list_head pointer address.

Changelog:

When reading the data by small chunks (i.e. byte by byte), the index (ppos) is
incremented by seq_read() directly and no "next" callback is called when going
to the next module.

Therefore, use ppos instead of m->private to deal with the fact that this index
is incremented directly to pass to the next module in seq_read() after the
buffer has been emptied.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
---
 fs/seq_file.c            |   45 ++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/seq_file.h |   20 ++++++++++++++++++++
 2 files changed, 64 insertions(+), 1 deletion(-)

Index: linux-2.6-lttng/fs/seq_file.c
===================================================================
--- linux-2.6-lttng.orig/fs/seq_file.c	2009-03-05 15:44:04.000000000 -0500
+++ linux-2.6-lttng/fs/seq_file.c	2009-03-05 15:44:36.000000000 -0500
@@ -671,5 +671,48 @@ struct list_head *seq_list_next(void *v,
 	++*ppos;
 	return lh == head ? NULL : lh;
 }
-
 EXPORT_SYMBOL(seq_list_next);
+
+struct list_head *seq_sorted_list_start(struct list_head *head, loff_t *ppos)
+{
+	struct list_head *lh;
+
+	list_for_each(lh, head)
+		if ((unsigned long)lh >= *ppos) {
+			*ppos = (unsigned long)lh;
+			return lh;
+		}
+	return NULL;
+}
+EXPORT_SYMBOL(seq_sorted_list_start);
+
+struct list_head *seq_sorted_list_start_head(struct list_head *head,
+		loff_t *ppos)
+{
+	struct list_head *lh;
+
+	if (!*ppos) {
+		*ppos = (unsigned long)head;
+		return head;
+	}
+	list_for_each(lh, head)
+		if ((unsigned long)lh >= *ppos) {
+			*ppos = (long)lh->prev;
+			return lh->prev;
+		}
+	return NULL;
+}
+EXPORT_SYMBOL(seq_sorted_list_start_head);
+
+struct list_head *seq_sorted_list_next(void *p, struct list_head *head,
+		loff_t *ppos)
+{
+	struct list_head *lh;
+	void *next;
+
+	lh = ((struct list_head *)p)->next;
+	next = (lh == head) ? NULL : lh;
+	*ppos = next ? ((unsigned long)next) : (-1UL);
+	return next;
+}
+EXPORT_SYMBOL(seq_sorted_list_next);
Index: linux-2.6-lttng/include/linux/seq_file.h
===================================================================
--- linux-2.6-lttng.orig/include/linux/seq_file.h	2009-03-05 15:44:04.000000000 -0500
+++ linux-2.6-lttng/include/linux/seq_file.h	2009-03-05 15:44:05.000000000 -0500
@@ -95,4 +95,24 @@ extern struct list_head *seq_list_start_
 extern struct list_head *seq_list_next(void *v, struct list_head *head,
 		loff_t *ppos);
 
+/*
+ * Helpers for iteration over a list sorted by ascending head pointer address.
+ * To be used in contexts where preemption cannot be disabled to insure to
+ * continue iteration on a modified list starting at the same location where it
+ * stopped, or at a following location. It insures that the lost information
+ * will only be in elements added/removed from the list between iterations.
+ * void *pos is only used to get the next list element and may not be a valid
+ * list_head anymore when given to seq_sorted_list_start() or
+ * seq_sorted_list_start_head().
+ */
+extern struct list_head *seq_sorted_list_start(struct list_head *head,
+		loff_t *ppos);
+extern struct list_head *seq_sorted_list_start_head(struct list_head *head,
+		loff_t *ppos);
+/*
+ * next must be called with an existing p node
+ */
+extern struct list_head *seq_sorted_list_next(void *p, struct list_head *head,
+		loff_t *ppos);
+
 #endif

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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