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Date: Wed, 27 Oct 2004 10:44:01 +0200
From: Ingo Molnar <mingo@elte.hu>
To: "K.R. Foley" <kr@cybsft.com>
Cc: linux-kernel@vger.kernel.org, Lee Revell <rlrevell@joe-job.com>,
       Rui Nuno Capela <rncbc@rncbc.org>, Mark_H_Johnson@Raytheon.com,
       Bill Huey <bhuey@lnxw.com>, Adam Heath <doogie@debian.org>,
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       Thomas Gleixner <tglx@linutronix.de>,
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       Fernando Pablo Lopez-Lezcano <nando@ccrma.Stanford.EDU>,
       Alexander Batyrshin <abatyrshin@ru.mvista.com>
Subject: Re: [patch] Real-Time Preemption, -RT-2.6.9-mm1-V0
Message-ID: <20041027084401.GA15989@elte.hu>
References: <20041022133551.GA6954@elte.hu> <20041022155048.GA16240@elte.hu> <20041022175633.GA1864@elte.hu> <20041025104023.GA1960@elte.hu> <417D4B5E.4010509@cybsft.com> <20041025203807.GB27865@elte.hu> <417E2CB7.4090608@cybsft.com> <20041027002455.GC31852@elte.hu> <417F16BB.3030300@cybsft.com> <20041027082831.GA15192@elte.hu>
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* Ingo Molnar <mingo@elte.hu> wrote:

> >  Oct 26 21:43:56 porky kernel: BUG: sleeping function called from 
> > invalid context amlat(3963) at kernel/mutex.c:28
> > Oct 26 21:43:56 porky kernel: in_atomic():1 [00000001], irqs_disabled():1
> > Oct 26 21:43:56 porky kernel:  [<c011c7da>] __might_sleep+0xca/0xe0 (12)
> > Oct 26 21:43:56 porky kernel:  [<c0137d89>] _mutex_lock+0x39/0x50 (36)
> > Oct 26 21:43:56 porky kernel:  [<c0137df6>]  _mutex_lock_irqsave+0x16/0x20 (24)
> > Oct 26 21:43:56 porky kernel:  [<c012977d>] __mod_timer+0x4d/0x1f0 (12)
> > Oct 26 21:43:56 porky kernel:  [<c01f6535>] rtc_do_ioctl+0x185/0x970 (44)
> 
> does the quick hack below help?

here's a more complete fix.

	Ingo

--- linux/drivers/char/rtc.c.orig
+++ linux/drivers/char/rtc.c
@@ -177,7 +177,7 @@ static unsigned long rtc_max_user_freq =
 /*
  * rtc_task_lock nests inside rtc_lock.
  */
-static DECLARE_SPINLOCK(rtc_task_lock);
+static DECLARE_RAW_SPINLOCK(rtc_task_lock);
 static rtc_task_t *rtc_callback = NULL;
 #endif
 
@@ -238,10 +238,17 @@ irqreturn_t rtc_interrupt(int irq, void 
 		rtc_irq_data |= (CMOS_READ(RTC_INTR_FLAGS) & 0xF0);
 	}
 
-	if (rtc_status & RTC_TIMER_ON)
+	if (rtc_status & RTC_TIMER_ON) {
+		spin_unlock (&rtc_lock);
+		/*
+		 * We do the mod_timer outside of the lock because
+		 * it may reschedule under PREEMPT_REALTIME. As long
+		 * as we read the flag race-free it is not a problem
+		 * if two mod_timer()s race:
+		 */
 		mod_timer(&rtc_irq_timer, jiffies + HZ/rtc_freq + 2*HZ/100);
-
-	spin_unlock (&rtc_lock);
+	} else
+		spin_unlock (&rtc_lock);
 
 	/* Now do the rest of the actions */
 	spin_lock(&rtc_task_lock);
@@ -1094,17 +1101,19 @@ static void rtc_dropped_irq(unsigned lon
 		return;
 	}
 
-	/* Just in case someone disabled the timer from behind our back... */
-	if (rtc_status & RTC_TIMER_ON)
-		mod_timer(&rtc_irq_timer, jiffies + HZ/rtc_freq + 2*HZ/100);
-
 	rtc_irq_data += ((rtc_freq/HZ)<<8);
 	rtc_irq_data &= ~0xff;
 	rtc_irq_data |= (CMOS_READ(RTC_INTR_FLAGS) & 0xF0);	/* restart */
 
 	freq = rtc_freq;
 
-	spin_unlock_irq(&rtc_lock);
+	/* Just in case someone disabled the timer from behind our back... */
+	if (rtc_status & RTC_TIMER_ON) {
+		spin_unlock_irq(&rtc_lock);
+		mod_timer(&rtc_irq_timer, jiffies + HZ/rtc_freq + 2*HZ/100);
+	} else
+		spin_unlock_irq(&rtc_lock);
+
 
 	printk(KERN_WARNING "rtc: lost some interrupts at %ldHz.\n", freq);
 
-
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