Re: [PATCH rdma-next] Revert "IB/core: Add flow control to the portmapper netlink calls"

[Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Fri, 2017-06-02 at 11:28 -0500, Shiraz Saleem wrote:
> On Wed, May 31, 2017 at 02:10:31PM -0600, Bart Van Assche wrote:
> > 
> > On Wed, 2017-05-31 at 12:42 -0500, Shiraz Saleem wrote:
> > > 
> > > > 
> > > > 5. I proposed a solution -> go and fix your user space program.
> > > 
> > > This is a kernel patch you are trying to revert, you are breaking
> > > existing
> > > kernel functionality.  Nothing to do with user space.
> > > 
> > > Bottom line, come up with a solution that will address both port
> > > mapper
> > > functionality and your issue.
> > 
> > Hello Shiraz,
> > 
> > Sorry that this means additional work for you, but I agree with
> > Leon that
> > user space software should not assume that netlink sockets are a
> > reliable
> > communication mechanism.
> 
> Hi Bart - Thank you for your response.
> 
> The original problem was that ibnl_unicast, which is used to send nl
> messages from 
> portmapper kernel space to user-space, would occasionally and
> momentarily fail under stress. 
> We could have retried the call for a certain amount of time, but
> since netlink_unicast has a 
> nonblock/block parameter, we chose to use the blocking option with a
> timeout. So we thought we 
> did account for deadlocks with this timeout.

Just to be clear, replacing a non-blocking and occasionally dropping
function with a blocking function but with a timeout does not actually
solve the problem, it merely moves the goal post out.  It is entirely
possible that you will have the problem again given sufficient load.

-- 
Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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