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    SubjectRe: [ofa-general] Re: dst_ifdown breaks infiniband?
    > Quoting David Miller <davem@davemloft.net>:
    > Subject: Re: [ofa-general] Re: dst_ifdown breaks infiniband?
    >
    > From: "Michael S. Tsirkin" <mst@dev.mellanox.co.il>
    > Date: Mon, 19 Mar 2007 00:42:34 +0200
    >
    > > > Quoting Michael S. Tsirkin <mst@dev.mellanox.co.il>:
    > > > Subject: Re: [ofa-general] Re: dst_ifdown breaks infiniband?
    > > >
    > > > > Quoting Eric W. Biederman <ebiederman@lnxi.com>:
    > > > > Subject: Re: [ofa-general] Re: dst_ifdown breaks infiniband?
    > > > >
    > > > > "Michael S. Tsirkin" <mst@dev.mellanox.co.il> writes:
    > > > >
    > > > > >> > Why is neighbour->dev changed here?
    > > > > >>
    > > > > >> It holds reference to device and prevents its destruction.
    > > > > >> If dst is held somewhere, we cannot destroy the device and deadlock
    > > > > >> while unregister.
    > > > > >
    > > > > > BTW, can this ever happen for the loopback device itself?
    > > > > > Is it ever unregistered?
    > > > >
    > > > > Well I don't think the loopback device is currently but as soon
    > > > > as we get network namespace support we will have multiple loopback
    > > > > devices and they will get unregistered when we remove the network
    > > > > namespace.
    > > >
    > > > Hmm. Then the code moving dst->dev to point to the loopback
    > > > device will have to be fixed too. I'll post a patch a bit later.
    > >
    > > Does this look sane (untested)?
    > >
    > > Signed-off-by: Michael S. Tsirkin <mst@dev.mellanox.co.il>
    >
    > You can't point it at NULL, we don't point it at loopback
    > just for fun.
    >
    > There can be asynchronous paths elsewhere in the networking still
    > referencing the neigh or dst and they will (correctly) feel free to
    > derefence whatever device is hanging there. So transitioning
    > to NULL is invalid.
    >
    > You guys will need to come up with a better solution for this silly
    > situation with network namespaces. Loopback is always available to
    > point dead routes and neighbour entries at, and this assumption is
    > massively rooted in the networking.

    Yes, I see this now.

    I guess it's best to focus on the original problem with dst_ifdown breaking
    infiniband for now.

    For that, we have to audit all the places where dst->neighbour is dereferenced for
    RCU safety, and this is already a massive task.

    --
    MST
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