Re: [PATCH v5 3/4] sample: rust_misc_device: Demonstrate additional get/set value functionality

[Lee Jones <lee@kernel.org>]

On Fri, 06 Dec 2024, Lee Jones wrote:

> On Fri, 06 Dec 2024, Greg KH wrote:
> 
> > On Fri, Dec 06, 2024 at 12:42:14PM +0000, Lee Jones wrote:
> > > Expand the complexity of the sample driver by providing the ability to
> > > get and set an integer.  The value is protected by a mutex.
> > > 
> > > Here is a simple userspace program that fully exercises the sample
> > > driver's capabilities.
> > 
> > nit, subject line should have "samples" not "sample" :)
> 
> Ack.
> 
> > > +    fn get_value(&self, mut writer: UserSliceWriter) -> Result<isize> {
> > > +        let guard = self.inner.lock();
> > > +        let value = guard.value;
> > > +
> > > +        // Refrain from calling write() on a locked resource
> > > +        drop(guard);
> > > +
> > > +        pr_info!("-> Copying data to userspace (value: {})\n", &value);
> > > +
> > > +        writer.write::<i32>(&value)?;
> > > +        Ok(0)
> > > +    }
> > 
> > I don't understand why you have to drop the mutex before calling
> > pr_info() and write (i.e. copy_to_user())?  It's a mutex, not a
> > spinlock, so you can hold it over that potentially-sleeping call, right?
> > Or is there some other reason why here?
> 
> This was a request from Alice to demonstrate how to unlock a mutex.

It's common practice to apply guards only around the protected value.

Why would this be different?

-- 
Lee Jones [李琼斯]