netdev - Re: possible circular locking dependency detected

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Message-ID: <20160902160026.GT2356@ZenIV.linux.org.uk>
Date:   Fri, 2 Sep 2016 17:00:26 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Rainer Weikusat <rweikusat@...eradapt.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        CAI Qian <caiqian@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Rainer Weikusat <rweikusat@...ileactivedefense.com>,
        Eric Sandeen <esandeen@...hat.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: possible circular locking dependency detected

On Fri, Sep 02, 2016 at 04:18:04PM +0100, Rainer Weikusat wrote:

> As far as I can tell, this should work as I can't currently imagine why
> a fs operation might end up binding a unix socket despite the idea to
> make af_unix.c yet more complicated in order to work around irregular
> behaviour of (as far as I can tell) a single filesystem (for which
> kern_path_create doesn't really mean kern_path_create

Bullshit.  kern_path_create() *does* mean the same thing in all cases.
Namely, find the parent, lock it and leave the final name component for
the create-type operation.  It sure as hell is not guaranteed to take
*all* locks that are going to be taken in process of mknod/mkdir/etc.
Never had been.

 and it has to work
> around that once it gets control) goes against all instincts I have in
> this area. If filesystems need to do arbitrary stuff when
> __sb_start_write is called for 'their' superblock, they should be able
> to do so directly.
> 
> At present, this is a theoretic concern as I can't (due to other work
> committments) put any non-cursory work into this before Sunday. There
> may also be other reasons why this idea is impractical or even
> unworkable.