Re: raw_spinlock_t rules

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 2010-09-19 at 15:09 +0200, Avi Kivity wrote:
> Some time ago, the i8259 emulation code in kvm was changed to use 
> raw_spinlock_t, as it was called in a preempt_disable() and 
> local_irq_disable() context, which doesn't work with preemptible 
> spinlocks used with CONFIG_PREEMPT_RT.
> 
> In Linux 2.6.37, the spinlock will no longer be taken in these contexts, 
> so I'd like to change it to a normal spinlock_t.  However, it is still 
> taken in a spin_lock_irq() context.
> 
> Is it okay to do this change?  I figured since spin_lock_irq() is part 
> of the spinlock infrastructure it might to the right thing.
> 

In PREEMPT_RT all interrupts (besides the timer) is converted to a
thread. The spin_lock_irq() will not disable interrupts in PREEMPT_RT,
because it is not needed (irqs are threaded and wont cause a deadlock if
they share the same spin_locks).

Note, spin_locks used by the timer will must be raw_spin_lock(), and
threaded context must disable interrupts for real before using it.

-- Steve