Date: Sat, 24 Jan 2009 11:20:43 +0100 From: Hans Petter Selasky <hselasky@c2i.net> To: freebsd-current@freebsd.org Cc: Alfred Perlstein <alfred@freebsd.org>, current@freebsd.org, Maksim Yevmenkin <maksim.yevmenkin@gmail.com> Subject: Re: panic: mutex Giant not owned at /usr/src/sys/kern/tty_ttydisc.c:1127 Message-ID: <200901241120.46889.hselasky@c2i.net> In-Reply-To: <bb4a86c70901240138g6a221fd4rbab3945193e4617@mail.gmail.com> References: <20090123154336.GJ60948@e.0x20.net> <200901240952.21670.hselasky@c2i.net> <bb4a86c70901240138g6a221fd4rbab3945193e4617@mail.gmail.com>
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Hi Maksim,
On Saturday 24 January 2009, Maksim Yevmenkin wrote:
> >> > I have a question. What is the following doing at the middle of the
> >> > ubt_detach():
> >> >
> >> > if (node != NULL)
> >> > NG_NODE_UNREF(node);
> >> >
> >> > If you say that:
> >> >
> >> > ng_rmnode_self(node);
> >> >
> >> > Cleans up the last reference?
> >>
> >> the complete code is
> >>
> >> node_p node = sc->sc_node;
> >>
> >> if (node != NULL) {
> >> sc->sc_node = NULL; <-- clear sc_node
> >>
> >> NG_NODE_SET_PRIVATE(node, NULL);
> >> NG_NODE_REALLY_DIE(node);
> >>
> >> NG_NODE_REF(node); <--- grab +1 reference
> >> ng_rmnode_self(node); <--- mark node as "dead", but not ensure its
> >> not free()d
> >> }
> >>
> >> /* bla, bla */
> >>
> >> if (node != NULL)
> >> NG_NODE_UNREF(node); <--- drop 1 reference and possibly free() node
> >
> > Yes, but you are already dropping an extra reference in ubt_shutdown().
> > What about that?
>
> shutdown method is called as part of ng_rmnode_self() and drop the
> reference that node was born with. the extra reference before
> ng_rmnode_self() is to ensure that node pointer is still valid after
> ng_rmnode_self() returns. otherwise there is a change that node
> pointer becomes invalid while after ng_rmnode_self() calls shutdown
> method.
I've now explicitly tested this and found that if I drop the node reference in
shutdown I end up with a zero node reference in detach! So the
NG_NODE_UNREF() should not be in the ubt_shutdown !!!
This is maybe the reason why Lars was getting a panic!
http://perforce.freebsd.org/chv.cgi?CH=156600
--HPS
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