From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: lockdep and threaded IRQs (was: ...)
Date: Fri, 27 Feb 2009 16:01:10 -0800
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On Fri, 27 Feb 2009 15:32:04 -0800
Andrew Morton <akpm@linux-foundation.org> 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.