netdev - RE: Netlink messages without NLM_F

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Message-ID: <2807E5FD2F6FDA4886F6618EAC48510E67CCE1CD@CRSMSX101.amr.corp.intel.com>
Date:   Wed, 7 Jun 2017 17:10:35 +0000
From:   "Weiny, Ira" <ira.weiny@...el.com>
To:     Leon Romanovsky <leon@...nel.org>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>
CC:     "Wan, Kaike" <kaike.wan@...el.com>,
        "Fleck, John" <john.fleck@...el.com>, Thomas Graf <tgraf@...g.ch>,
        Doug Ledford <dledford@...hat.com>,
        Jiri Pirko <jiri@...lanox.com>,
        RDMA mailing list <linux-rdma@...r.kernel.org>,
        linux-netdev <netdev@...r.kernel.org>
Subject: RE: Netlink messages without NLM_F_REQUEST flag

> 
> 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@...idianresearch.com> 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.
> 

Reference?

>From my understanding, NLM_F_REQUEST | NLM_F_ACK is simply requesting an ack from the kernel on a request.  In our case the message is a response to the kernel request.

Ira