linux-kernel - Re: [PATCH V23 2/3] misc: dcc: Add driver support for Data Capture and Compare unit(DCC)

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Message-ID: <2023061548-subtly-cackle-8be2@gregkh>
Date:   Thu, 15 Jun 2023 16:06:33 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Souradeep Chowdhury <quic_schowdhu@...cinc.com>
Cc:     Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Rob Herring <robh+dt@...nel.org>, Alex Elder <elder@...e.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        Sibi Sankar <quic_sibis@...cinc.com>,
        Rajendra Nayak <quic_rjendra@...cinc.com>
Subject: Re: [PATCH V23 2/3] misc: dcc: Add driver support for Data Capture
 and Compare unit(DCC)

On Thu, Jun 15, 2023 at 07:17:34PM +0530, Souradeep Chowdhury wrote:
> > > > > +static ssize_t ready_read(struct file *filp, char __user *userbuf,
> > > > > +			  size_t count, loff_t *ppos)
> > > > > +{
> > > > > +	int ret = 0;
> > > > > +	char *buf;
> > > > > +	struct dcc_drvdata *drvdata = filp->private_data;
> > > > > +
> > > > > +	mutex_lock(&drvdata->mutex);
> > > > > +
> > > > > +	if (!is_dcc_enabled(drvdata)) {
> > > > > +		ret = -EINVAL;
> > > > > +		goto out_unlock;
> > > > > +	}
> > > > > +
> > > > > +	if (!FIELD_GET(BIT(1), readl(drvdata->base + dcc_status(drvdata->mem_map_ver))))
> > > > > +		buf = "Y\n";
> > > > > +	else
> > > > > +		buf = "N\n";
> > > > > +out_unlock:
> > > > > +	mutex_unlock(&drvdata->mutex);
> > > > > +
> > > > > +	if (ret < 0)
> > > > > +		return -EINVAL;
> > > > > +	else
> > > > 
> > > > You do the "lock, get a value, unlock, do something with the value"
> > > > thing a bunch, but what prevents the value from changing after the lock
> > > > happens?  So why is the lock needed at all?
> > > 
> > > The lock is used to prevent concurrent accesses of the drv_data when
> > > scripts are being run from userspace.
> > 
> > How would that matter?  The state can change instantly after the lock is
> > given up, and then the returned value is now incorrect.  So no need for
> > a lock at all as you really aren't "protecting" anything, or am I
> > missing something else?
> 
> This lock is needed to protect the access to the global instance of drv_data
> structure instantiated at probe time within each individual callbacks of
> debugfs.

What exactly are you "protecting" here that could change in a way that
cause a problem?

You aren't returning a value that is ever guaranteed to be "correct"
except that it happened sometime in the past, it might be right anymore.

thanks,

greg k-h