Re: scheduler support to lock user processes out?

[Andrew Doran <ad%netbsd.org@localhost>]

On Thu, Feb 21, 2008 at 06:27:41AM -0700, M. Warner Losh wrote:

> Correct.  FreeBSD's giant won't solve this problem, nor will NetBSD's
> biglock when it covers less of the kernel.

We already have drivers that don't take kernel_lock and it's easy to switch
it off. Even for the vast majority of drivers that run rith it, acquring
kernel_lock doesn't have the intended effect.

The lesser hack of suspending user activity can be handled by adding another
LWP state. We can't abuse the SUSPENDED or STOP states because they are used
by applications. We can do something like:

- Offline all but the current CPU.

- Disable preemption. At this point there is only one user LWP left running
  in the system, the LWP that requested the suspend.

- Transition all but non-critical LWPs to the new state PAUSED (or whatever
  it's called). This would need to include a number of kernel threads, for
  example those handling ktrace. It would need to skip others, like the
  cross call threads.

Andrew