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.
Signed-off-by: NRobin Holt <holt@sgi.com>
Cc: Jack Steiner <steiner@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/misc/sgi-xp/xpc_uv.c

@@ -1428,7 +1428,6 @@ xpc_handle_notify_mq_msg_uv(struct xpc_partition *part,
 	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_channel *ch, void *payload)
 			       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 = {