Re: netlink: Add netlink_bound helper and use it in netlink_getname

From: Tejun Heo
Date: Sat Sep 26 2015 - 15:46:04 EST

Hello, Herbert.

On Sun, Sep 27, 2015 at 03:41:10AM +0800, Herbert Xu wrote:
> Thread 1 Thread 2
> sendmsg getsockname
> netlink_autobind netlink_getname
> Thread 2 should not have to do anything special to guarantee that
> getsockname does not return garbage. It must either be the bound
> portid if the autobind completed in thread 1 and is visible or it
> should return zero.
> As it stands thread 2 may see a portid belonging to somebody else
> if it catches the autobind in thread 1 trying different portids
> while roving.

If the fact that thread 1 finished autobind isn't visible to thread 2,
it's valid for getsockname to return zero. No ordering between the
two operations is defined. If the fact that thread 1 finished
autobind is visible to thread 2, ordering is defined and because
ordering is transitive, by that very ordering, the port number is
visible to thread 2 too as long as thread 1 does proper barriering.


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