[PATCH] rtc: fix deadlock

[Ingo Molnar]

* Mikael Pettersson <mikpe@xxxxxxxx> wrote:

> Since 2.6.27-rc1 my Core2Duo has been getting sporadic oopses
> from hpet_rtc_interrupt, usually during shutdown or reboot,
> but occasionally also early in init. Today I finally managed
> to capture one via a serial cable:
> 
> INIT: version 2.86 booting
> 		Welcome to Fedora Core
> 		Press 'I' to enter interactive startup.
> BUG: NMI Watchdog detected LOCKUP on CPU0, ip c0117092, registers:
> Modules linked in: ehci_hcd uhci_hcd usbcore
> 
> Pid: 311, comm: nash-hotplug Not tainted (2.6.27-rc4 #1)
> EIP: 0060:[<c0117092>] EFLAGS: 00000097 CPU: 0
> EIP is at hpet_rtc_interrupt+0x2d2/0x310
> EAX: 00000000 EBX: 00000002 ECX: 00000046 EDX: 00000002
> ESI: 000000a6 EDI: ffff8e25 EBP: 00000008 ESP: f7bd7f28
>  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
> Process nash-hotplug (pid: 311, ti=f7bd6000 task=f7b70460 task.ti=f7bd6000)
> Stack: f7bd7f6c c0139cc0 00000000 c035ba04 00000000 00000000 00000000 00000000 
>        00000000 00000000 00000000 00000000 00000000 f7b845a0 00000000 00000000 
>        00000008 c01478a8 c035bf80 f7b845a0 c035bfb0 00000008 c0148f71 00000400 
> Call Trace:
>  [<c0139cc0>] hrtimer_run_pending+0x20/0x90
>  [<c01478a8>] handle_IRQ_event+0x28/0x50
>  [<c0148f71>] handle_edge_irq+0xa1/0x120
>  [<c010615b>] do_IRQ+0x3b/0x70
>  [<c0113225>] smp_apic_timer_interrupt+0x55/0x80
>  [<c0103c4f>] common_interrupt+0x23/0x28
>  [<c02c0000>] unix_release_sock+0xc0/0x220
>  =======================
> Code: 89 44 24 18 0f b6 c2 e8 5d 74 0c 00 8b 0d d8 9c 3b c0 89 44 24 1c 8b 44 24 0c 48 89 44 24 20 e9 84 fd ff ff 90 8d 74 26 00 f3 90 <a1> 80 ba 35 c0 29 f8 83 f8 01 76 f2 e9 e1 fe ff ff 90 8d 74 26 
> 
> This points to the following loop in hpet_rtc_interrupt:
> 
> 0xc0117090 <hpet_rtc_interrupt+720>:    pause  
> 0xc0117092 <hpet_rtc_interrupt+722>:    mov    0xc035ba80,%eax
> 0xc0117097 <hpet_rtc_interrupt+727>:    sub    %edi,%eax
> 0xc0117099 <hpet_rtc_interrupt+729>:    cmp    $0x1,%eax
> 0xc011709c <hpet_rtc_interrupt+732>:    jbe    0xc0117090 <hpet_rtc_interrupt+720>
> 
> Note: 0xc035ba80 == &jiffies
> 
> This loop originates from asm-generic/rtc.h:get_rtc_time()
> 
> 		while (jiffies - uip_watchdog < 2*HZ/100) {
> 			barrier();
> 			cpu_relax();
> 		}
> 
> Note: HZ == CONFIG_HZ == 100
> 
> The bug may not originate from the 2.6.27-rc series as I only recently 
> enabled HPET in this machine's kernels (not due to HPET problems, it 
> inherited its .config way back from an older machine w/o HPET).

argh, that loop in asm-generic/rtc.h:get_rtc_time looks extremely 
fragile, we'll lock up if it's ever called with hardirqs off!

Does the patch below do the trick?

	Ingo

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