linux-kernel - Re: [PATCH 5/9] usb: fix empty-body warning in sysfs.c

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Date:   Tue, 21 Apr 2020 11:20:53 +1000
From:   NeilBrown <neilb@...e.de>
To:     Alan Stern <stern@...land.harvard.edu>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        linux-input@...r.kernel.org, Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        linux-nfs@...r.kernel.org,
        Johannes Berg <johannes@...solutions.net>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>, linux-nvdimm@...ts.01.org,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, target-devel@...r.kernel.org,
        Zzy Wysm <zzy@...wysm.com>
Subject: Re: [PATCH 5/9] usb: fix empty-body warning in sysfs.c

On Sat, Apr 18 2020, Alan Stern wrote:

> On Sat, 18 Apr 2020, Matthew Wilcox wrote:
>
>> On Sat, Apr 18, 2020 at 11:41:07AM -0700, Randy Dunlap wrote:
>> > +++ linux-next-20200327/drivers/usb/core/sysfs.c
>> > @@ -1263,7 +1263,7 @@ void usb_create_sysfs_intf_files(struct
>> >  	if (!alt->string && !(udev->quirks & USB_QUIRK_CONFIG_INTF_STRINGS))
>> >  		alt->string = usb_cache_string(udev, alt->desc.iInterface);
>> >  	if (alt->string && device_create_file(&intf->dev, &dev_attr_interface))
>> > -		;	/* We don't actually care if the function fails. */
>> > +		do_empty(); /* We don't actually care if the function fails. */
>> >  	intf->sysfs_files_created = 1;
>> >  }
>> 
>> Why not just?
>> 
>> +	if (alt->string)
>> +		device_create_file(&intf->dev, &dev_attr_interface);
>
> This is another __must_check function call.
>
> The reason we don't care if the call fails is because the file
> being created holds the USB interface string descriptor, something
> which is purely informational and hardly ever gets set (and no doubt
> gets used even less often).
>
> Is this another situation where the comment should be expanded and the 
> code modified to include a useless test and cast-to-void?
>
> Or should device_create_file() not be __must_check after all?

One approach to dealing with __must_check function that you don't want
to check is to cause failure to call
   pr_debug("usb: interface descriptor file not created");
or similar.  It silences the compiler, serves as documentation, and
creates a message that is almost certainly never seen.

This is what I did in drivers/md/md.c...

	if (mddev->kobj.sd &&
	    sysfs_create_group(&mddev->kobj, &md_bitmap_group))
		pr_debug("pointless warning\n");

(I give better warnings elsewhere - I must have run out of patience by
 this point).

NeilBrown

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