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Date:   Mon, 7 Jun 2021 14:08:27 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Leon Romanovsky <leon@kernel.org>
Cc:     Doug Ledford <dledford@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Kees Cook <keescook@chromium.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Adit Ranadive <aditr@vmware.com>,
        Ariel Elior <aelior@marvell.com>,
        Christian Benvenuti <benve@cisco.com>,
        clang-built-linux@googlegroups.com,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        Devesh Sharma <devesh.sharma@broadcom.com>,
        Gal Pressman <galpress@amazon.com>,
        linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org,
        Michal Kalderon <mkalderon@marvell.com>,
        Mike Marciniszyn <mike.marciniszyn@cornelisnetworks.com>,
        Mustafa Ismail <mustafa.ismail@intel.com>,
        Naresh Kumar PBS <nareshkumar.pbs@broadcom.com>,
        Nelson Escobar <neescoba@cisco.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Potnuri Bharat Teja <bharat@chelsio.com>,
        Selvin Xavier <selvin.xavier@broadcom.com>,
        Shiraz Saleem <shiraz.saleem@intel.com>,
        VMware PV-Drivers <pv-drivers@vmware.com>,
        Yishai Hadas <yishaih@nvidia.com>,
        Zhu Yanjun <zyjzyj2000@gmail.com>
Subject: Re: [PATCH rdma-next v1 10/15] RDMA/cm: Use an attribute_group on
 the ib_port_attribute intead of kobj's
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On Mon, Jun 07, 2021 at 02:37:16PM +0300, Leon Romanovsky wrote:
> On Mon, Jun 07, 2021 at 01:22:26PM +0200, Greg KH wrote:
> > 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? 
> 
> On 32 bits, and yes, we have a customer who still uses such system.

So you will see what, a "tear"?  Or a stale value?

> > As these are per-port, do multiple threads try to increment these at
> > the same time?  
> 
> Yes, CM connection can be seen as thread. Bottom line everything in parallel.
> 
> > And even if they do, what happens if one is 'dropped' somehow because of this?
> 
> Probably nothing, we increment the statistics only.

So you are hitting cache lines for no good reason, probably not a good
idea, you are wasting cpu cycles for nothing :(

thanks,

greg k-h