linux-kernel - Re: [patch 3/7] procfs: Add ability to plug in auxiliary fdinfo providers

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Date:	Fri, 27 Jul 2012 10:08:38 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>
Subject: Re: [patch 3/7] procfs: Add ability to plug in auxiliary fdinfo providers

>>> +static void *seq_next(struct seq_file *m, void *p, loff_t *pos)
>>> +{
>>> +	struct proc_fdinfo_extra *extra = m->private;
>>> +	void *v = NULL;
>>> +
>>> +	if (extra->driver) {
>>> +		int ret = 0;
>>> +
>>> +		if (*pos == 0) {
>>> +			v = extra->driver->ops->start(m, pos);
>>> +			if (v) {
>>> +				ret = extra->driver->ops->show(m, v);
>>
>> Why is it necessary to call ->show here? The logic should be
>>
>> seq_start = (pos == 0 ? nop : extra->start)
>> seq_next  = (pos == 0 ? extra->start : extra->next)
>> seq_stop  = (pos == 0 ? nop : extra->stop)
>> seq_show  = (pos == 0 ? proc_show : extra->show)
>>
>> Or I'm missing something?
> 
> Well, I thought about it as two sequences -- first is procfs
> seq-file, which prints out a general header, and second
> is extra fdinfo provider.
> 
> Everything starts with printing procfs header
> 
> seq_start -> seq_show (prints "pos:\t%lli\nflags:\t0%o\n") ->
> seq_next -> (if have extra driver we do extra's start/show at first,

Why show? The generic seq_file.c will call ->next (which will result in
extra->start), then ->show (which _should_ result in extra->show) thus
providing correct output.

> then next and etc). In other words general header should be
> shown always even if extra's start() fails.
> 
> 	Cyrill
> .
> 

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