spin_lock_irqsave and ISP interrupts on OMAP 3530

I'm hoping I'm just not understanding something and this isn't as broke as I fear it is.

I'm using the ISP on an OMAP3530, linux 2.6.32. I've got some kernel code I want to lock in both the process context and the ISP interrupt context, so I'm using a spinlock. As I understand it, locking the spinlock should disable all interrupts. However, I'm still seeing the ISP interrupt happen when I've got the spinlock locked in the process context.

i.e. somewhere I've got the code below, and my Warning is tripping. Is this broken, or am I not understanding something?

int am_i_locked = 0;
spinlock_t my_spinlock = SPIN_LOCK_UNLOCKED;

void foo(void) {
    /* function could be called from process context, or could be 
called from omap34xx_isp_isr (interrupt context) */
    unsigned long flags;
    spin_lock_irqsave(&my_spinlock, flags);
    WARN_ON(am_i_locked);
    am_i_locked++;

    /* do some stuff */

    am_i_locked--;
    spin_unlock_irqrestore(&my_spinlock, flags);
}

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