netdev - Re: [PATCH 2/2] af_unix: split 'u->readlock' into two: 'iolock' and 'bindlock'

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Date:   Fri, 2 Sep 2016 13:00:27 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Miller <davem@...emloft.net>
Cc:     Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Rainer Weikusat <rweikusat@...ileactivedefense.com>,
        Eric Dumazet <edumazet@...gle.com>, Willy Tarreau <w@....eu>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/2] af_unix: split 'u->readlock' into two: 'iolock' and 'bindlock'

On Fri, Sep 2, 2016 at 12:15 PM, David Miller <davem@...emloft.net> wrote:
>
> The main thing I was concerned about was an I/O path that really
> expects the socket's hash not to change for whatever reason, but even
> all of the unix_find_other() calls are done outside of the mutex
> already.

That's my read of it too. I did actually go through the effort on
trying to follow all the locking in there yesterday, so while I would
want an AF_UNIX person to double-check me, I think it's fine.

The real locking seem to be the u->lock spinlock (taken by
unix_state_lock() and friends).

The bindlock is only about serializing the binders, which do things
that can sleep (notably the mknod of the unix socket node in the
filesystem).

The one thing I looked at and wasn't sure about was
unix_stream_connect(), which creates a *new* unix domain socket and
binds it to the old one without holding the bindlock. Note that it
never did hold the lock, so that's not a new thing, but it worries me
a bit.

I *think* it's ok, because I think this all happens before that new
socket is actually reachable, so it cannot race against somebody else
doing a bind. And my patch doesn't actually change anything in this
area, so I'm not making it worse. But it was the one thing I reacted
to when going through the locking there.

(Well, not the "one thing". The other thing I reacted to was the
overall reaction that none of this is documented, and the locking was
clearly not very clear).

                 Linus