On Fri, 27 Feb 2009 15:32:04 -0800
Andrew Morton [off-list ref] wrote:

Why does this function:

static irqreturn_t powerbutton_irq(int irq, void *dev_id)
{
	int err;
	u8 value;

#ifdef CONFIG_LOCKDEP
	/* WORKAROUND for lockdep forcing IRQF_DISABLED on us, which
	 * we don't want and can't tolerate.  Although it might be
	 * friendlier not to borrow this thread context...
	 */
	local_irq_enable();
#endif

	err = twl4030_i2c_read_u8(TWL4030_MODULE_PM_MASTER, &value,
				  STS_HW_CONDITIONS);
	if (!err)  {
		input_report_key(powerbutton_dev, KEY_POWER,
				 value & PWR_PWRON_IRQ);
	} else {
		dev_err(dbg_dev, "twl4030: i2c error %d while reading TWL4030"
			" PM_MASTER STS_HW_CONDITIONS register\n", err);
	}

	return IRQ_HANDLED;
}

Which is connected up via this statement:

	err = request_irq(irq, powerbutton_irq,
			IRQF_TRIGGER_FALLING | IRQF_TRIGGER_RISING,
			"twl4030-pwrbutton", NULL);

reenable local interrupts?

ah, OK, twl4030_i2c_read_u8() does i2c I/O.

Can't do that.  If some random process currently holds
mutex_lock(&twl->xfer_lock) and an interrupt occurs then this interrupt
handler will try to acquire mutex_lock(&twl->xfer_lock).  Deadlock.