Re: [PATCH] lockdep: Avoid /proc/lockdep & lock_stat infinite output

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2007-10-09 at 02:30 +0100, Al Viro wrote:
> On Mon, Oct 08, 2007 at 06:15:51PM -0700, Tim Pepper wrote:
> > 
> > When a read() requests an amount of data smaller than the amount of data
> > that the seq_file's foo_show() outputs, the output starts looping and
> > outputs the "stuck" element's data infinitely.  There may be multiple
> > sequential calls to foo_start(), foo_next()/foo_show(), and foo_stop()
> > for a single open with sequential read of the file.  The _start() does not
> > have to start with the 0th element and _show() might be called multiple
> > times in a row for the same element for a given open/read of the seq_file.
> >  
> >  static void *l_start(struct seq_file *m, loff_t *pos)
> >  {
> > -	struct lock_class *class = m->private;
> > +	struct lock_class *class;
> > +	loff_t i = 0;
> >  
> > -	if (&class->lock_entry == all_lock_classes.next)
> > +	if (*pos == 0)
> >  		seq_printf(m, "all lock classes:\n");
> 
> Do not generate output outside of ->show() and you won't have these
> problems.  That's where your infinite output crap comes from.
> 
> IOW, NAK - fix the underlying problem.


FWIW I had to do Tim's bits too. Just moving all output from the start
into the show method didn't fix it.

Signed-off-by: Tim Pepper <lnxninja@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/lockdep_proc.c |   69 +++++++++++++++++++++++---------------------------
 1 file changed, 33 insertions(+), 36 deletions(-)

Index: linux-2.6/kernel/lockdep_proc.c
===================================================================
--- linux-2.6.orig/kernel/lockdep_proc.c
+++ linux-2.6/kernel/lockdep_proc.c
@@ -23,32 +23,25 @@
 
 #include "lockdep_internals.h"
 
-static void *l_next(struct seq_file *m, void *v, loff_t *pos)
+static void *l_start(struct seq_file *m, loff_t *pos)
 {
-	struct lock_class *class = v;
-
-	(*pos)++;
-
-	if (class->lock_entry.next != &all_lock_classes)
-		class = list_entry(class->lock_entry.next, struct lock_class,
-				  lock_entry);
-	else
-		class = NULL;
-	m->private = class;
+	struct lock_class *class;
+	int i = 0;
 
-	return class;
+	list_for_each_entry(class, &all_lock_classes, lock_entry) {
+		if (i++ == *pos)
+			return class;
+	}
+	return NULL;
 }
 
-static void *l_start(struct seq_file *m, loff_t *pos)
+static void *l_next(struct seq_file *m, void *v, loff_t *pos)
 {
-	struct lock_class *class = m->private;
-
-	if (&class->lock_entry == all_lock_classes.next)
-		seq_printf(m, "all lock classes:\n");
-
-	return class;
+	(*pos)++;
+	return l_start(m, pos);
 }
 
+
 static void l_stop(struct seq_file *m, void *v)
 {
 }
@@ -101,10 +94,16 @@ static void print_name(struct seq_file *
 static int l_show(struct seq_file *m, void *v)
 {
 	unsigned long nr_forward_deps, nr_backward_deps;
-	struct lock_class *class = m->private;
+	struct lock_class *class = v;
 	struct lock_list *entry;
 	char c1, c2, c3, c4;
 
+	if (WARN_ON(class == NULL))
+		return 0;
+
+	if (&class->lock_entry == all_lock_classes.next)
+		seq_printf(m, "all lock classes:\n");
+
 	seq_printf(m, "%p", class->key);
 #ifdef CONFIG_DEBUG_LOCKDEP
 	seq_printf(m, " OPS:%8ld", class->ops);
@@ -522,28 +521,19 @@ static void seq_header(struct seq_file *
 static void *ls_start(struct seq_file *m, loff_t *pos)
 {
 	struct lock_stat_seq *data = m->private;
+	struct lock_stat_data *iter;
 
-	if (data->iter == data->stats)
-		seq_header(m);
-
-	if (data->iter == data->iter_end)
-		data->iter = NULL;
+	iter = data->iter + *pos;
+	if (iter >= data->iter_end)
+		iter = NULL;
 
-	return data->iter;
+	return iter;
 }
 
 static void *ls_next(struct seq_file *m, void *v, loff_t *pos)
 {
-	struct lock_stat_seq *data = m->private;
-
 	(*pos)++;
-
-	data->iter = v;
-	data->iter++;
-	if (data->iter == data->iter_end)
-		data->iter = NULL;
-
-	return data->iter;
+	return ls_start(m, pos);
 }
 
 static void ls_stop(struct seq_file *m, void *v)
@@ -553,8 +543,15 @@ static void ls_stop(struct seq_file *m, 
 static int ls_show(struct seq_file *m, void *v)
 {
 	struct lock_stat_seq *data = m->private;
+	struct lock_stat_data *iter = v;
+
+	if (WARN_ON(iter == NULL))
+		return 0;
+
+	if (iter == data->iter)
+		seq_header(m);
 
-	seq_stats(m, data->iter);
+	seq_stats(m, iter);
 	return 0;
 }

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