linux-kernel - Re: [PATCH v2 02/16] bus: mhi: core: Add support for registering MHI controllers

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Message-ID: <20200211192236.GB1962867@kroah.com>
Date:   Tue, 11 Feb 2020 11:22:36 -0800
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     arnd@...db.de, smohanad@...eaurora.org, jhugo@...eaurora.org,
        kvalo@...eaurora.org, bjorn.andersson@...aro.org,
        hemantk@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/16] bus: mhi: core: Add support for registering MHI
 controllers

On Wed, Feb 12, 2020 at 12:41:47AM +0530, Manivannan Sadhasivam wrote:
> Hi Greg,
> 
> On Thu, Feb 06, 2020 at 05:56:06PM +0100, Greg KH wrote:
> > On Fri, Jan 31, 2020 at 07:19:55PM +0530, Manivannan Sadhasivam wrote:
> > > +static void mhi_release_device(struct device *dev)
> > > +{
> > > +	struct mhi_device *mhi_dev = to_mhi_device(dev);
> > > +
> > > +	if (mhi_dev->ul_chan)
> > > +		mhi_dev->ul_chan->mhi_dev = NULL;
> > 
> > That looks really odd.  Why didn't you just drop the reference you
> > should have grabbed here for this pointer?  You did properly increment
> > it when you saved it, right?  :)
> > 
> 
> Well, there is no reference count (kref) exist for mhi_dev.

Then something is wrong with your model :(

You can't save pointers off to things without reference counting, that
is going to cause you real problems.  See the coding style document for
all the details.

> And we really needed to NULL the mhi_dev to avoid any dangling
> reference to it.

Again, that's not how to do this correctly.

> The reason for not having kref is that, each mhi_dev will be used by
> maximum of 2 channels only. So thought that refcounting is not needed.
> Please correct me if I'm wrong.

Please read section 11 of Documentation/process/coding-style.rst

thanks,

greg k-h