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

[Greg Kroah-Hartman]

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