linux-kernel - Re: X fails to start with latest Linus git

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Date:	Sun, 27 Jan 2008 13:41:20 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Kevin Winchester <kjwinchester@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	John Stultz <johnstul@...ibm.com>
Subject: Re: X fails to start with latest Linus git


* Ingo Molnar <mingo@...e.hu> wrote:

> it seems you've got hpet active by default:
> 
>  /sys/devices/system/clocksource/clocksource0/current_clocksource:
>  hpet
>  /sys/devices/system/clocksource/clocksource0/available_clocksource:
>  hpet acpi_pm pit jiffies tsc

btw., it's quite mysterious how this can lock up. ktime_get() on a hpet 
clocksource does a very straightforward:

 static cycle_t read_hpet(void)
 {
         return (cycle_t)hpet_readl(HPET_COUNTER);
 }

which is a very simple thing:

c0119470 <hpet_readl>:
c0119470:       8b 15 b8 b4 ae c0       mov    0xc0aeb4b8,%edx
c0119476:       55                      push   %ebp
c0119477:       89 e5                   mov    %esp,%ebp
c0119479:       5d                      pop    %ebp
c011947a:       01 d0                   add    %edx,%eax
c011947c:       8b 00                   mov    (%eax),%eax
c011947e:       c3                      ret

basically just a memory access to an ioremap()-ed area.

hm, perhaps it's due to the xtime lock dependency:

        do {
                seq = read_seqbegin(&xtime_lock);
                getnstimeofday(ts);
                tomono = wall_to_monotonic;

        } while (read_seqretry(&xtime_lock, seq));

perhaps your system somehow generates a printk from within an xtime_lock 
locked section?

	Ingo
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