linux-kernel - Re: [git pull] gadgetfs fixes

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Date:	Sun, 15 Mar 2015 09:09:46 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Alexander Holler <holler@...oftware.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: [git pull] gadgetfs fixes

On Sun, Mar 15, 2015 at 09:50:01AM +0100, Alexander Holler wrote:

> Hmm, a year ago or so I've stumbled over the fact that the owner
> might be necessary for correct entries in sysfs (that was mtd). I've
> no idea if that's true here too but it might be worse to mention it.

There are two mechanisms.  One keeps the filesystem your file is on pinned
while the file is opened.  That works for all filesystems, and it's enough
when the methods of that file are in the module defining that filesystem.
In cases when that is not enough (e.g. character device living on ext3 -
it's nice to have ext3 pinned down, but you want the driver that defined
that device to be pinned as well) you can ask to pin a given module for
as long as the file is opened.  That's what file_operations ->owner gives.
Your example (mtd creating a file on sysfs) also needs that - you want
mtd pinned, not just sysfs.

gadgetfs, however, has filesystem defined by the same module.  So pinning
filesystem_type down is enough - nothing extra is needed, same as for e.g.
a regular file on ext3.
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