Subject: Re: Time to fix a 25 year old misdesign
To: None <>
From: Lennart Augustsson <>
List: tech-kern
Date: 10/15/2000 19:36:09
Lennart Augustsson wrote:

> The old UNIX device driver interface is fundamentally broken.  The
> problem is the following
>    The driver routine open() is called every time the device
>    is opened, but close() is only called on the last close.

> How should it be fixed?  Well, the only way I can see is to change
> when the close() routine is called for character device drivers.
> Doing that would be a huge amount of work, so instead I suggest that
> we add a routine to the struct cdevsw.  This routine will have the
> same type as the close() routine, but will be called on every close().

Thinking some more about my proposal I realize that it is flawed.
The problem is that it is not only the open() system call that generates
an extra reference to a device vnode, but also for().
This means that with my scheme there would (potentially) be more
calls to dev_close() than dev_open(), which is also broken.

This could be fixed by introducing a device method that is called when
the reference count is increased (e.g. by fork).  But now its starts to
get unelegant...


        -- Lennart