linux-kernel - Re: [PATCH v2 14/16] net: qrtr: Add MHI transport layer

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Date:   Tue, 4 Feb 2020 13:49:16 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     gregkh@...uxfoundation.org, 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,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 14/16] net: qrtr: Add MHI transport layer

Hi Jakub,

On Mon, Feb 03, 2020 at 10:12:25AM -0800, Jakub Kicinski wrote:
> On Fri, 31 Jan 2020 19:20:07 +0530, Manivannan Sadhasivam wrote:
> > +/* From QRTR to MHI */
> > +static void qcom_mhi_qrtr_ul_callback(struct mhi_device *mhi_dev,
> > +				      struct mhi_result *mhi_res)
> > +{
> > +	struct qrtr_mhi_dev *qdev = dev_get_drvdata(&mhi_dev->dev);
> > +	struct qrtr_mhi_pkt *pkt;
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&qdev->ul_lock, flags);
> > +	pkt = list_first_entry(&qdev->ul_pkts, struct qrtr_mhi_pkt, node);
> > +	list_del(&pkt->node);
> > +	complete_all(&pkt->done);
> > +
> > +	kref_put(&pkt->refcount, qrtr_mhi_pkt_release);
> 
> Which kref_get() does this pair with?
> 
> Looks like qcom_mhi_qrtr_send() will release a reference after
> completion, too.
> 

Yikes, there is some issue here...

Acutally the issue is not in what you referred above but the overall kref
handling itself. Please see below.

kref_put() should be present in qcom_mhi_qrtr_ul_callback() as it will
decrement the refcount which got incremented in qcom_mhi_qrtr_send(). It
should be noted that kref_init() will fix the refcount to 1 and kref_get() will
increment to 2. So for properly releasing the refcount to 0, we need to call
kref_put() twice.

So if all goes well, the refcount will get decremented twice in
qcom_mhi_qrtr_ul_callback() as well as in qcom_mhi_qrtr_send() and we are good.

But, if the transfer has failed ie., when qcom_mhi_qrtr_ul_callback() doesn't
get called, then we are leaking the refcount. I need to rework the kref handling
code in next iteration.

Thanks for triggering this!

Regards,
Mani

> > +	spin_unlock_irqrestore(&qdev->ul_lock, flags);
> > +}