raw_spinlock_t rules

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.

Quoted 5 lines · Avi Kivity

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.

Quoted 7 lines

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.