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Date:   Mon, 7 Jun 2021 09:32:10 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Leon Romanovsky <leon@kernel.org>,
        Doug Ledford <dledford@redhat.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 02/15] RDMA/core: Replace the ib_port_data
 hw_stats pointers with a ib_port pointer
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On Mon, Jun 07, 2021 at 12:23:23PM +0200, Greg KH wrote:
> >  static int add_port(struct ib_core_device *coredev, int port_num)
> >  {
> >  	struct ib_device *device = rdma_device_to_ibdev(&coredev->dev);
> > @@ -1171,6 +1177,8 @@ static int add_port(struct ib_core_device *coredev, int port_num)
> >  		setup_hw_stats(device, p, port_num);
> >  
> >  	list_add_tail(&p->kobj.entry, &coredev->port_list);
> > +	if (device->port_data && is_full_dev)
> > +		device->port_data[port_num].sysfs = p;
> 
> You are saving off a pointer to a reference counted structure without
> incrementing the reference count on it?  

This storage borrows another reference count, primarily because there
is no locking to read/write .sysfs. It is a fairly common idiom.

You can see it in the free path:

		port->ibdev->port_data[port->port_num].sysfs = NULL;
	kobject_put(&port->kobj);  // port == p above

Due to the lack of locks the whole external thing is arranged so that
the 3 users of .sysfs are sequenced properly around
setup_port()/destroy_port() using other external locks.

Adding more refs without also adding locking is just confusing what
the data protection model is. This is a borrowed ref and access is
only allowed when other locking is properly sequencing it with the ref
owner's manipulation of .sysfs.

Eg I would reject some code sequence like this:

	port->ibdev->port_data[port->port_num].sysfs = NULL;
	kobject_put(&port->kobj);  // one for .sysfs
	kobject_put(&port->kobj);  // one for our stack

As being pretty bogus.

Jason