[9fans] MIPS _xdec kernel vs. user land

[erik quanstrom]

i know it's not used much, but this is very curious, and if the kernel
version is correct, the user space version needs correcting, too.

the rb kernel version starts with

        TEXT    _xdec(SB), $0
                SYNC

why is a sync necessary?  i would think, if LL/SC don't do what
they do atomicly, then we've already lost, otherwise no sync is
necessary.

- erik