Linux kernel interrupt handler mutex protection?

Do I need to protect my interrupt handler being called many times for the same interrupt?

Given the following code, I am not sure on the system calls I should make. I am getting rare, random dead-locks with this current implementation :-

void interrupt_handler(void)
{
    down_interruptible(&sem);  // or use a lock here ?

    clear_intr(); // clear interrupt source on H/W

    wake_up_interruptible(...);

    up(&sem); // unlock?

    return IRQ_HANDLED;
}

void set/clear_intr()
{
    spin_lock_irq(&lock);
    RMW(x); // set/clear a bit by read/modify/write the H/W interrupt routing register
    spin_unlock_irq(&lock);
}

void read()
{
    set_intr();  // same as clear_intr, but sets a bit
    wait_event_interruptible(...);
}

1. Should interrupt_handler:down_interruptible be spin_lock_irq / spin_lock_irqsave / local_irq_disable?
2. Should set/clear_intr:spin_lock_irq be spin_lock_irqsave / local_irq_disable?
3. Can it (H/W -> kernel -> driver handler) keep generating/getting interrupts until its cleared? Can the interrupt_handler keep getting called while within it?
4. If as currently implemented the interrupt handler is reentrant then will it block on the down_interruptible?

From LDD3 :-

must be reentrant—it must be capable of running in more than one context at the same time.

Edit 1) after some nice help, suggestions are :-

1. remove down_interruptible from within interrupt_handler
2. Move spin_lock_irq outside set/clear methods (no need for spin_lock_irqsave you say?) I really don't see the benefit to this?!

Code :-

void interrupt_handler(void)
{
    read_reg(y); // eg of other stuff in the handler

    spin_lock_irq(&lock);

    clear_intr(); // clear interrupt source on H/W

    spin_unlock_irq(&lock);

    wake_up_interruptible(...);

    return IRQ_HANDLED;
}

void set/clear_intr()
{
    RMW(x);
}

void read()
{
    error_checks(); // eg of some other stuff in the read method

    spin_lock_irq(&lock);

    set_intr();  // same as clear_intr, but sets a bit

    spin_unlock_irq(&lock);

    wait_event_interruptible(...);

    // more code here...
}

Edit2) After reading some more SO posts : reading Why kernel code/thread executing in interrupt context cannot sleep? which links to Robert Loves article, I read this :

some interrupt handlers (known in Linux as fast interrupt handlers) run with all interrupts on the local processor disabled. This is done to ensure that the interrupt handler runs without interruption, as quickly as possible. More so, all interrupt handlers run with their current interrupt line disabled on all processors. This ensures that two interrupt handlers for the same interrupt line do not run concurrently. It also prevents device driver writers from having to handle recursive interrupts, which complicate programming.

And I have fast interrupts enabled (SA_INTERRUPT)! So no need for mutex/locks/semaphores/spins/waits/sleeps/etc/etc!