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

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

[Tim Pepper]

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.

Signed-off-by: Tim Pepper <lnxninja@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>

---

Assuming people are fine with this, it should probably find its way
to stable.

If you haven't seen the infinite output: it's easy to trigger with a
simple 'cat /proc/lockdep' generally for me, a cat /proc/lock_stat piped
to a file or for either of them a dd with the default bs=512 (or smaller)
should do the job also.

With this change to the lock_stat handler the data->iter member no longer
attempts to hold state across calls, so it could be taken out of the
lock_stat_seq struct and replace by a local variable in each function
but that isn't a clear win to me so I just left it.

--- linux-2.6.23-rc9.orig/kernel/lockdep_proc.c
+++ linux-2.6.23-rc9/kernel/lockdep_proc.c
@@ -34,19 +34,23 @@ static void *l_next(struct seq_file *m, 
 				  lock_entry);
 	else
 		class = NULL;
-	m->private = class;
 
 	return class;
 }
 
 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");
 
+	list_for_each_entry(class, &all_lock_classes, lock_entry) {
+		if (i++ == *pos)
+			return class;
+	}
+	return NULL;
-	return class;
 }
 
 static void l_stop(struct seq_file *m, void *v)
@@ -101,7 +105,7 @@ 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;
 
@@ -523,12 +527,15 @@ static void *ls_start(struct seq_file *m
 {
 	struct lock_stat_seq *data = m->private;
 
-	if (data->iter == data->stats)
-		seq_header(m);
+	data->iter = data->stats;
+	data->iter += *pos;
 
-	if (data->iter == data->iter_end)
+	if (data->iter >= data->iter_end)
 		data->iter = NULL;
 
+	if (data->iter == data->stats)
+		seq_header(m);
+
 	return data->iter;
 }
 

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

[Al Viro]

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.

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

[Tim Pepper]

On Tue 09 Oct at 02:30:11 +0100 viro@ftp.linux.org.uk said:
> On Mon, Oct 08, 2007 at 06:15:51PM -0700, Tim Pepper wrote:
> > 
> > -	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.

Aaah...OK.  Can we add something like the following then:




Document that output must only come from _show() and SEQ_START_TOKEN is how
a _start() indicates a header is to be printed.

Signed-off-by: Tim Pepper <lnxninja@linux.vnet.ibm.com>
Cc: Al Viro <viro@ftp.linux.org.uk>

---

--- linux-2.6.orig/include/linux/seq_file.h
+++ linux-2.6.23-rc9/include/linux/seq_file.h
@@ -36,9 +36,10 @@ ssize_t seq_read(struct file *, char __u
 loff_t seq_lseek(struct file *, loff_t, int);
 int seq_release(struct inode *, struct file *);
 int seq_escape(struct seq_file *, const char *, const char *);
+
+/* these may only be called from a (*show) function */
 int seq_putc(struct seq_file *m, char c);
 int seq_puts(struct seq_file *m, const char *s);
-
 int seq_printf(struct seq_file *, const char *, ...)
 	__attribute__ ((format (printf,2,3)));
 
@@ -48,6 +49,11 @@ int single_open(struct file *, int (*)(s
 int single_release(struct inode *, struct file *);
 int seq_release_private(struct inode *, struct file *);
 
+/*
+ * return SEQ_START_TOKEN in your (*start) function and test for
+ * (v == SEQ_START_TOKEN) in * your (*show) funtion in order to
+ * print a header before your seq data
+ */
 #define SEQ_START_TOKEN ((void *)1)
 
 /*

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

[Peter Zijlstra]

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

[Tim Pepper]

On Tue 09 Oct at 13:14:49 +0200 a.p.zijlstra@chello.nl said:
> FWIW I had to do Tim's bits too. Just moving all output from the start
> into the show method didn't fix it.

Yes.  The way the original lockdep_proc.c code was doing its pointers
around its seq data was definitely wrong, regardless of the output
outside of _show().

> -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);
>  }

Isn't this is going to make l_start/l_next() approach O(n^2) when they can
be more like O(n)?  It appears to be the case that _next() will always
get a valid *v, so you can just step immediately to the next element.
The _start() seems to be the only place where you'd actually need to
search based on *pos.

The below moves the headers out of the _start() functions, but by using
SEQ_START_TOKEN (as appears to be the trend in other seq_file users)
to differentiate.  This means *pos==0 then is the header and *pos==1..n
are the lock class elements 0..(n-1), which again appears to be what
others are doing.

================================================================

Both /proc/lockdep and /proc/lock_stat output may loop infinitely.

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.

Also header output should not be happening in _start().  All output should
be in _show(), which SEQ_START_TOKEN is meant to help.  Having output in
_start() may also negatively impact seq_file's seq_read() and traverse()
accounting.

Signed-off-by: Tim Pepper <lnxninja@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Al Viro <viro@ftp.linux.org.uk>

---

Compared to my previous version this now also has output only happening
in _show().  Compared to Peter's version with output only in _show(),
this is more efficient in its _next().


--- linux-2.6.23-rc9.orig/kernel/lockdep_proc.c
+++ linux-2.6.23-rc9/kernel/lockdep_proc.c
@@ -25,28 +25,38 @@
 
 static void *l_next(struct seq_file *m, void *v, loff_t *pos)
 {
-	struct lock_class *class = v;
+	struct lock_class *class;
 
 	(*pos)++;
 
+	if (v == SEQ_START_TOKEN)
+		class = m->private;
+	else {
+		class = v;
+
+		if (class->lock_entry.next != &all_lock_classes)
+			class = list_entry(class->lock_entry.next,
+					   struct lock_class, lock_entry);
+		else
+			class = NULL;
+	}
-	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;
 
 	return class;
 }
 
 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)
-		seq_printf(m, "all lock classes:\n");
+	if (*pos == 0)
+		return SEQ_START_TOKEN;
 
+	list_for_each_entry(class, &all_lock_classes, lock_entry) {
+		if (++i == *pos)
+		return class;
+	}
+	return NULL;
-	return class;
 }
 
 static void l_stop(struct seq_file *m, void *v)
@@ -101,10 +111,15 @@ 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 (v == SEQ_START_TOKEN) {
+		seq_printf(m, "all lock classes:\n");
+		return 0;
+	}
+
 	seq_printf(m, "%p", class->key);
 #ifdef CONFIG_DEBUG_LOCKDEP
 	seq_printf(m, " OPS:%8ld", class->ops);
@@ -523,10 +538,11 @@ static void *ls_start(struct seq_file *m
 {
 	struct lock_stat_seq *data = m->private;
 
-	if (data->iter == data->stats)
-		seq_header(m);
+	if (*pos == 0)
+		return SEQ_START_TOKEN;
 
-	if (data->iter == data->iter_end)
+	data->iter = data->stats + *pos;
+	if (data->iter >= data->iter_end)
 		data->iter = NULL;
 
 	return data->iter;
@@ -538,8 +554,13 @@ static void *ls_next(struct seq_file *m,
 
 	(*pos)++;
 
+	if (v == SEQ_START_TOKEN)
+		data->iter = data->stats;
+	else {
+		data->iter = v;
+		data->iter++;
+	}
+
-	data->iter = v;
-	data->iter++;
 	if (data->iter == data->iter_end)
 		data->iter = NULL;
 
@@ -552,9 +573,11 @@ 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;
+	if (v == SEQ_START_TOKEN)
+		seq_header(m);
+	else
+		seq_stats(m, v);
 
-	seq_stats(m, data->iter);
 	return 0;
 }