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
