Re: [PATCH rdma-next v1 10/15] RDMA/cm: Use an attribute_group on the ib_port_attribute intead of kobj's

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jun 07, 2021 at 01:51:36PM +0300, Leon Romanovsky wrote:
> On Mon, Jun 07, 2021 at 12:25:03PM +0200, Greg KH wrote:
> > On Mon, Jun 07, 2021 at 11:17:35AM +0300, Leon Romanovsky wrote:
> > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > 
> > > This code is trying to attach a list of counters grouped into 4 groups to
> > > the ib_port sysfs. Instead of creating a bunch of kobjects simply express
> > > everything naturally as an ib_port_attribute and add a single
> > > attribute_groups list.
> > > 
> > > Remove all the naked kobject manipulations.
> > 
> > Much nicer.
> > 
> > But why do you need your counters to be atomic in the first place?  What
> > are they counting that requires this?  Given that they are just a
> > statistic for userspace, making them be a u64 should work just the same,
> > right?
> 
> The statistic counters are per-port, while the cm.c flows run in
> asynchronically in parallel for every CM connection.
> 
> We need atomic variable to ensure that "write to u64" is not
> interrupted.

On what system is "write to u64" interruptable?  As these are per-port,
do multiple threads try to increment these at the same time?  And even
if they do, what happens if one is 'dropped' somehow because of this?
It's just a userspace statistic counter, what relies on this being
exact?

thanks,

greg k-h