linux-kernel - Re: [PATCH] xen/debugfs: Check debugfs initialization before using it

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Message-ID: <CABawtvNUBwTVzFfR4UotCexARN4+As5wKEPyGmRzN7-xZ6J00w@mail.gmail.com>
Date:	Mon, 9 Dec 2013 21:42:23 +0800
From:	Ethan Zhao <ethan.kernel@...il.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	konrad.wilk@...cle.com, raghavendra.kt@...ux.vnet.ibm.com,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen/debugfs: Check debugfs initialization before using it

Greg,
    I am the man who built a Xen dom0, but couldn't see debugfs
directory and files as expected. there is no warning or tip for me to
enable the CONFIG_DEBUG_FS=y in .config , it cost me minutes to figure
out what's
the matter. and I know should check defugfs config and initialization as
zswap_debugfs, tracer_debugfs ,rproc_debugfs did. Is it useless ? if
it could save me just 1 minute next time ?

Thanks,
Ethan

On Mon, Dec 9, 2013 at 7:17 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Mon, Dec 09, 2013 at 05:57:22PM +0800, Ethan Zhao wrote:
>> On Mon, Dec 9, 2013 at 4:55 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
>> > On Mon, Dec 09, 2013 at 04:44:05PM +0800, Ethan Zhao wrote:
>> >> On Mon, Dec 9, 2013 at 4:25 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
>> >> > On Mon, Dec 09, 2013 at 09:43:16AM +0800, Ethan Zhao wrote:
>> >> >> On Sun, Dec 8, 2013 at 10:01 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
>> >> >> > On Sun, Dec 08, 2013 at 07:31:02PM +0800, ethan.zhao wrote:
>> >> >> >> Should check debugfs initialization with debugfs_initialized() before using it,
>> >> >> >> Because if it isn't initialized, the return value of fake debugfs_create_dir() etc
>> >> >> >> functions would be ERR_PTR(-ENODEV), checking with NULL will not work.
>> >> >> >
>> >> >> > So?  It should "just work" without this check, right?  What happens if
>> >> >> > your patch isn't applied and debugfs isn't enabled?
>> >> >>
>> >> >> If debugfs isn't  configured, debugfs_initialized() and other
>> >> >> functions are defined as following,
>> >> >>
>> >> >> static inline bool debugfs_initialized(void)
>> >> >> {
>> >> >> return false;
>> >> >> }
>> >> >>
>> >> >> static inline struct dentry *debugfs_create_file(const char *name, umode_t mode,
>> >> >> struct dentry *parent, void *data,
>> >> >> const struct file_operations *fops)
>> >> >> {
>> >> >> return ERR_PTR(-ENODEV);
>> >> >> }
>> >> >>
>> >> >> static inline struct dentry *debugfs_create_dir(const char *name,
>> >> >> struct dentry *parent)
>> >> >> {
>> >> >> return ERR_PTR(-ENODEV);
>> >> >> }
>> >> >>
>> >> >> And the checking code in xen\debugfs.c xen_init_debugfs() will not
>> >> >> work, the return value is not NULL.
>> >> >> d_xen_debug = debugfs_create_dir("xen", NULL);
>> >> >>
>> >> >> if (!d_xen_debug)
>> >> >> pr_warning("Could not create 'xen' debugfs directory\n");
>> >> >
>> >> > Which is just fine, what is wrong with this?
>> >>
>> >> If we no check with debugfs_initialized(),  the above code should be
>> >> if  (!d_xen_debug || IS_ERR (d_xen_debug))
>> >>     pr_warning("Could not create 'xen' debugfs directory\n");
>> >
>> > No, you don't want to print out that message if debugfs is not enabled.
>> >
>> > You really don't want to print anything out, as what can a user do about
>> > this?
>> >
>> Yep, should output a warning about "Debugfs is not configured and enabled"
>
> No, not at all, it's not an issue you need to warn about because it was
> explicitly told to be that way by the person who built that kernel.
>
> It's not an error, please don't treat it as such.
>
> greg k-h
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