Re: Netlink messages without NLM_F_REQUEST flag

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Wed, Jun 07, 2017 at 08:00:37PM +0300, Leon Romanovsky wrote:
> On Wed, Jun 07, 2017 at 10:47:50AM -0600, Jason Gunthorpe wrote:
> > On Wed, Jun 07, 2017 at 07:43:44PM +0300, Leon Romanovsky wrote:
> > > On Wed, Jun 7, 2017 at 7:37 PM, Jason Gunthorpe
> > > <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org> wrote:
> > > > On Wed, Jun 07, 2017 at 07:19:01PM +0300, Leon Romanovsky wrote:
> > > >> It makes me wonder if it is expected behavior for
> > > >> ibnl_rcv_reply_skb() to handle !NLM_F_REQUEST messages and do we
> > > >> really need it? What are the scenarios?  In my use case, which is
> > > >> for sure different from yours, I'm always setting NLM_F_REQUEST
> > > >> while communicating with kernel.
> > > >
> > > > If I recall the user space SA code issues REQUESTS from the kernel to
> > > > userspace, so userspace returns with the response format. This is
> > > > abnormal for netlink hence the special function.
> > >
> > > In netlink semantics, kernel side is supposed to send netlink
> > > notification message and userspace is supposed to send REQUEST.
> >
> > That pattern is for async communications, the SA stuff needs a sync
> > protocol, unfortunately.
> 
> There is special flag NLM_F_ACK for it and userspace will set
> NLM_F_REQUEST | NLM_F_ACK once synchronization is needed.

AFAIK, that is different, that is acking and retriggering a single shot
notification, not completing a kernel initiated handshake.

Jason
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