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    Subject[patch 8/8] X86: UV - XPC receive message reuse triggers invalid BUG_ON().

    This was a difficult bug to trip. XPC was in the middle of sending an
    acknowledgement for a received message.

    In xpc_received_payload_uv():
    ..
    ret = xpc_send_gru_msg(ch->sn.uv.cached_notify_gru_mq_desc, msg,
    sizeof(struct xpc_notify_mq_msghdr_uv));
    if (ret != xpSuccess)
    XPC_DEACTIVATE_PARTITION(&xpc_partitions[ch->partid], ret);

    msg->hdr.msg_slot_number += ch->remote_nentries;

    at the point in xpc_send_gru_msg() where the hardware has dispatched the
    acknowledgement, the remote side is able to reuse the message structure
    and send a message with a different slot number. This problem is made
    worse by interrupts.

    The adjustment of msg_slot_number and the BUG_ON in
    xpc_handle_notify_mq_msg_uv() which verifies the msg_slot_number is
    consistent are only used for debug purposes. Since a fix for this that
    preserves the debug functionality would either have to infringe upon
    the payload or allocate another structure just for debug, I decided to
    remove it entirely.

    To: Ingo Molnar <mingo@elte.hu>
    To: tglx@linutronix.de
    Signed-off-by: Robin Holt <holt@sgi.com>
    Cc: Jack Steiner <steiner@sgi.com>
    Cc: linux-kernel@vger.kernel.org

    ---
    drivers/misc/sgi-xp/xpc_uv.c | 3 ---
    1 file changed, 3 deletions(-)
    Index: linux-x86/drivers/misc/sgi-xp/xpc_uv.c
    ===================================================================
    --- linux-x86.orig/drivers/misc/sgi-xp/xpc_uv.c 2009-11-18 22:16:45.000000000 -0600
    +++ linux-x86/drivers/misc/sgi-xp/xpc_uv.c 2009-11-18 22:16:45.000000000 -0600
    @@ -1428,7 +1428,6 @@ xpc_handle_notify_mq_msg_uv(struct xpc_p
    msg_slot = ch_uv->recv_msg_slots +
    (msg->hdr.msg_slot_number % ch->remote_nentries) * ch->entry_size;

    - BUG_ON(msg->hdr.msg_slot_number != msg_slot->hdr.msg_slot_number);
    BUG_ON(msg_slot->hdr.size != 0);

    memcpy(msg_slot, msg, msg->hdr.size);
    @@ -1652,8 +1651,6 @@ xpc_received_payload_uv(struct xpc_chann
    sizeof(struct xpc_notify_mq_msghdr_uv));
    if (ret != xpSuccess)
    XPC_DEACTIVATE_PARTITION(&xpc_partitions[ch->partid], ret);
    -
    - msg->hdr.msg_slot_number += ch->remote_nentries;
    }

    static struct xpc_arch_operations xpc_arch_ops_uv = {


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