[openib-general] Re: [PATCH (updated)] ipoib: dont lock tx on completion

[Roland Dreier]

    Michael> We are now only looking at tx_tail and
    Michael> netif_queue_stopped outside the lock (tx_head is only
    Michael> used inside the lock):

Thanks, that helps.

    Michael> We have a write barrier in between, so its guaranteed
    Michael> tx_tail is written before we test netif_queue_stopped.

    Michael> On the other hand, the send code has a barrier after it
    Michael> stops the interface, so this bit is written before we
    Michael> test tx_tail the second time.

    Michael> So, in the scenario where send stops the interface,
    Michael> either the completion code will see it stopped, or the
    Michael> send code will see tx tail updated.

You may be right but I'm having a hard time convincing myself.  I
don't think just the write barriers are enough -- they just mean that
the CPU where the writes are being done will complete the writes in
order.  Without a read barrier, the CPU doing the tests can execute
the read whenever it wants.  I've seen the PPC 970 move reads absurdly
far, and the compiler can reorder things even more.

 - R.