Subject: MP and device drivers
To: None <>
From: Andrew Doran <>
List: tech-smp
Date: 09/03/2006 16:02:38
Hi all,

I would like to gate entry to character and block drivers so that we acquire
the kernel_lock if the driver is not marked as MP safe.

Here are my initial thoughts:

- Make callout_init() take a flag indicating MP-safety. We'd take the
  kernel_lock just before final dispatch of the callout.

- Put wrappers around the devsw entry points (e.g. bdev_strategy(dev, bp)).
  In the MP case these would check d_flag and maybe grab the kernel_lock.
  We'd need to assert that no other locks are already held.

- Modify generic soft interrupts and hard interupts not to take the kernel
  lock if the handler is marked as MP safe.

- Take the lock in shutdown hooks and perhaps other minor entry points.
  As others have mentioned these should be proper driver entry points but
  that's a whole other can of worms.

There are some problems with this approach:

- There are multiple places in each driver where we have to specify 'MP
  safe'. Ideally we would stash this information somewhere else, like
  cfattach or the lkm info.

- This doesn't handle network drivers and 'oddball' drivers, but that's
  beyond what I'm trying to do.

Can you see other problems with this approach? Thoughts?