Re: [PATCH] kvm: Drop obsolete cpu_get/put in make_all_cpus_request

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jul 21, 2009 at 10:24:08AM +0200, Jan Kiszka wrote:
> Marcelo Tosatti wrote:
> > Jan,
> > 
> > This was suggested but we thought it might be safer to keep the
> > get_cpu/put_cpu pair in case -rt kernels require it (which might be
> > bullshit, but nobody verified).
> 
> -rt stumbles over both patterns (that's why I stumbled over it in the
> first place: get_cpu disables preemption, but spin_lock is a sleeping
> lock under -rt) and actually requires requests_lock to become
> raw_spinlock_t. Reordering get_cpu and spin_lock would be another
> option, but not really a gain for both scenarios.

I see.

> So unless there is a way to make the whole critical section preemptible
> (thus migration-agnostic), I think we can micro-optimize it like this.

Can't you switch requests_lock to be raw_spinlock_t then? (or whatever
is necessary to make it -rt compatible).