Re: Exposing semaphores

[Greg KH <greg@kroah.com>]

On Tue, Apr 27, 2021 at 06:18:52AM -0400, Sven Van Asbroeck wrote:
> Hi Carlos,
> 
> On Mon, Apr 26, 2021 at 8:02 PM Carlos <carloscarral13@gmail.com> wrote:
> >
> > I recently began assessing what functionality from the kernel is missing
> > for implementing a basic scull driver.
> >
> > Semaphores are the first missing primitive that caught my attention, and
> > being such an important part of the kernel itself, I will make exposing
> > this functionality my first priority instead.
> 
> Isn't scull's semaphore used exclusively in mutex mode, i.e.
> initialized using init_MUTEX?
> 
> If so, prefer "struct mutex" instead. At the time scull was written
> (over 15 years ago) it did not yet exist.
> A Rust-for-Linux abstraction is already available:
> https://github.com/Rust-for-Linux/linux/blob/rust/rust/kernel/sync/mutex.rs
> 
> This is before my time, but I believe that the use of "bare" struct
> semaphores in drivers used to be a common anti-pattern, often
> resulting in buggy code and/or performance issues - most semaphores
> have been replaced by conceptually safer, higher-level primitives, and
> today one needs a valid excuse to use them in new or even refactored
> driver code. But I do not know the full story.

I agree, never try to ab-use a semaphore in the way we did it in the
book, that is not a good "pattern" to follow for the majority of kernel
drivers.

A simple mutex should be all that is necessary.

But also, while the CUSE driver has a lot of history, it really was only
created as an example to show how things could work, it isn't all that
useful in the real world :)

thanks,

greg k-h