Need some help in interpreting a code snippet

[Nagaprabhanjan Bellari <nagp.knb@xxxxxxxxx>]

The code snippet is in linux/kernel/softirq.c:__do_softirq():

==

    do {

        if (pending & 1) {

            int preempt_count = preempt_count();

            h->action(h);

            if (preempt_count != preempt_count()) {

                printk(KERN_CRIT "BUG: softirq: %p, %x,%x\n",

                    h->action, preempt_count, preempt_count());

                dump_crit_stack();

            }

            rcu_bh_qsctr_inc(cpu);

        }

==

What I am trying to understand is the preempt_count check after h->action(h); statement. Can somebody tell me why such a prempt_count mismatch check is made and why a mismatch is critical?

Thanks,

-nagp