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

On Sat, Sep 26, 2015 at 03:45:54PM -0400, Tejun Heo wrote:

Hello, Herbert.

On Sun, Sep 27, 2015 at 03:41:10AM +0800, Herbert Xu wrote:

quoted

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.

If the autobind is not complete then netlink_getname must return
zero rather than some garbage portid that belongs to somebody
else's socket.  That's what we did before any of this lockless
code was introduced.

If you don't check nlk->bound then you may return garbage.

Cheers,
-- 
Email: Herbert Xu [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
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