From owner-freebsd-hackers  Fri May 10 13:13:31 2002
Delivered-To: freebsd-hackers@freebsd.org
Received: from viola.sinor.ru (viola.sinor.ru [217.70.106.9])
	by hub.freebsd.org (Postfix) with ESMTP
	id 0E71637B405; Fri, 10 May 2002 13:13:09 -0700 (PDT)
Received: from tlg5-ppp59.sibnet.ru (tlg5-ppp59.sibnet.ru [217.70.97.60])
	by viola.sinor.ru (8.9.3/8.9.3) with ESMTP id DAA05084;
	Sat, 11 May 2002 03:13:01 +0700
Date: Sat, 11 May 2002 03:12:56 +0700 (NOVST)
From: "Semen A. Ustimenko" <semenu@FreeBSD.org>
X-X-Sender: semenu@def.the.net
To: freebsd-fs@FreeBSD.org
Cc: freebsd-hackers@FreeBSD.org, "Flood, Jim" <Jim.Flood@acirro.com>
Subject: NULLFS-related possible deadlock + fix proposal
Message-ID: <20020511005932.S1705-100000@def.the.net>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: owner-freebsd-hackers@FreeBSD.ORG
Precedence: bulk
List-ID: <freebsd-hackers.FreeBSD.ORG>
List-Archive: <http://docs.freebsd.org/mail/> (Web Archive)
List-Help: <mailto:majordomo@FreeBSD.ORG?subject=help> (List Instructions)
List-Subscribe: <mailto:majordomo@FreeBSD.ORG?subject=subscribe%20freebsd-hackers>
List-Unsubscribe: <mailto:majordomo@FreeBSD.ORG?subject=unsubscribe%20freebsd-hackers>
X-Loop: FreeBSD.ORG

Hi!

Preface:

Same directory is null-mounted to "/mnt" and "/mnt2". The directory
contain "dir/foofile". Two processes concurently lookup "/mnt/dir/foofile"
and "/mnt2/dir/foofile".

Action:

P1:
  in lookup():
    in VOP_LOOKUP(dvp (== "/mnt/dir"), "foofile"):
      in null_lookup():
        in null_node_create():
          in malloc() | getnewvnode() | somewhere():
            in tsleep() -> P1 is preempted by P2

(Note, that "foofile"'s lower vnode is locked by P1, "dir"'s lower
vnode is unlocked, thus "/mnt2/dir" is also unlocked)

P2:
  in lookup():
    in VOP_LOOKUP(dvp (== "/mnt2/dir"), "foofile):
      in null_lookup():
        in VOP_LOOKUP(lowerdvp, "foofile"):
           in tsleep(), waiting for "foofile"'s lower vnode, held by P1

(Note, the "/mnt2/dir"'s vnode and thus its lower vnode is still locked by
P2, the "foofile"'s lower vnode is locked by P1)

P1:
  in lookup():
    in vrele(dvp (== "/mnt/dir")):
      in vn_lock(dvp):
        in tsleep(), waiting for "/mnt/dir"'s lower vnode, held by P2

DEADLOCK...

Analysis:

The lookup() routine can call vrele(), in its turn vrele() can vn_lock()
parent directory, while holding lock on file from this directory. This
isn't a problem for nonstacking FSes as vrele() will only vn_lock if it
were the last reference.

For NULLFS this is a problem because completely different vnodes can share
the lock structure.

Solution:

Make vn_lock() in vrele() lock vnode only LK_THISLAYER. Obviously, the
NULLFS and other stacking FSes will have to deal with this in their
VOP_INACTIVE() handlers. This changes won't touch real FSes as they ignore
the LK_THISLAYER, don't they?

Bye!


To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message