Re: [PATCH 1/7] gnss: add GNSS receiver subsystem

[Johan Hovold <johan@kernel.org>]

On Wed, Apr 25, 2018 at 12:56:45PM +0200, Johan Hovold wrote:
> On Wed, Apr 25, 2018 at 10:56:49AM +0200, Greg Kroah-Hartman wrote:
> > On Tue, Apr 24, 2018 at 06:34:52PM +0200, Johan Hovold wrote:

> > > +static int gnss_open(struct inode *inode, struct file *file)
> > > +{
> > > +	struct gnss_device *gdev;
> > > +	int ret = 0;
> > > +
> > > +	gdev = container_of(inode->i_cdev, struct gnss_device, cdev);
> > > +
> > > +	get_device(&gdev->dev);
> > > +
> > > +	nonseekable_open(inode, file);
> > > +	file->private_data = gdev;
> > > +
> > > +	down_write(&gdev->rwsem);
> > 
> > Just curious, why a rwsem?  They can be slower than a normal semaphore,
> > is this really a contentious lock?
> 
> I use the rwsem to deal with hotplugging; the underlying device can go
> away at any time and the core makes sure that no further calls into the
> corresponding driver is made once all currently executing callbacks
> return.

I just did find one access to the gnss ops which was unsafe however; the
existence check for a write_raw callback in write() needs to be replaced
by a device flag.

Thanks,
Johan