linux-kernel - Re: [BUG] rtc uie emulation deadlocks

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Message-Id: <20080819005320.00075dba.akpm@linux-foundation.org>
Date:	Tue, 19 Aug 2008 00:53:20 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Russell King <rmk+lkml@....linux.org.uk>
Cc:	a.zummo@...ertech.it,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	David Brownell <david-b@...bell.net>
Subject: Re: [BUG] rtc uie emulation deadlocks

On Mon, 11 Aug 2008 14:43:39 +0100 Russell King <rmk+lkml@....linux.org.uk> wrote:

> With CONFIG_RTC_INTF_DEV_UIE_EMUL=y, hwclock deadlocks in rtc_read_time
> due to:
> 
> static long rtc_dev_ioctl(struct file *file,
>                 unsigned int cmd, unsigned long arg)
> {
>         err = mutex_lock_interruptible(&rtc->ops_lock);
>         if (err)
>                 return err;
> ...
>         switch (cmd) {
> #ifdef CONFIG_RTC_INTF_DEV_UIE_EMUL
>         case RTC_UIE_OFF:
>                 clear_uie(rtc);
>                 break;
> 
>         case RTC_UIE_ON:
>                 err = set_uie(rtc);
> #endif
> }
> 
> static int set_uie(struct rtc_device *rtc)
> {
>         err = rtc_read_time(rtc, &tm);
> ...
> 
> int rtc_read_time(struct rtc_device *rtc, struct rtc_time *tm)
> {
>         err = mutex_lock_interruptible(&rtc->ops_lock);
> 
> So we try to take the same lock twice...  Not good.
> 
> The locking looks a rather random in there, so I'm not going to try to
> create a patch.
> 

Looks like this was caused by

commit 5ad31a575157147b43fa84ef1e21471661653878
Author: David Brownell <david-b@...bell.net>
Date:   Wed Jul 23 21:30:33 2008 -0700

    rtc: remove BKL for ioctl()
    

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