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    Subject[PATCH] rtc: fix deadlock

    * Mikael Pettersson <mikpe@it.uu.se> 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

    ----------------->
    From 2273cc870b52a7ed09eb225142a6db97299e4f39 Mon Sep 17 00:00:00 2001
    From: Ingo Molnar <mingo@elte.hu>
    Date: Sat, 23 Aug 2008 17:59:07 +0200
    Subject: [PATCH] rtc: fix deadlock

    if get_rtc_time() is _ever_ called with IRQs off, we deadlock badly
    in it, waiting for jiffies to increment.

    So make the code more robust by doing an explicit mdelay(20).

    This solves a very hard to reproduce/debug hard lockup reported
    by Mikael Pettersson.

    Reported-by: Mikael Pettersson <mikpe@it.uu.se>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    include/asm-generic/rtc.h | 12 ++++--------
    1 files changed, 4 insertions(+), 8 deletions(-)

    diff --git a/include/asm-generic/rtc.h b/include/asm-generic/rtc.h
    index be4af00..71ef3f0 100644
    --- a/include/asm-generic/rtc.h
    +++ b/include/asm-generic/rtc.h
    @@ -15,6 +15,7 @@
    #include <linux/mc146818rtc.h>
    #include <linux/rtc.h>
    #include <linux/bcd.h>
    +#include <linux/delay.h>

    #define RTC_PIE 0x40 /* periodic interrupt enable */
    #define RTC_AIE 0x20 /* alarm interrupt enable */
    @@ -43,7 +44,6 @@ static inline unsigned char rtc_is_updating(void)

    static inline unsigned int get_rtc_time(struct rtc_time *time)
    {
    - unsigned long uip_watchdog = jiffies;
    unsigned char ctrl;
    unsigned long flags;

    @@ -53,19 +53,15 @@ static inline unsigned int get_rtc_time(struct rtc_time *time)

    /*
    * read RTC once any update in progress is done. The update
    - * can take just over 2ms. We wait 10 to 20ms. There is no need to
    + * can take just over 2ms. We wait 20ms. There is no need to
    * to poll-wait (up to 1s - eeccch) for the falling edge of RTC_UIP.
    * If you need to know *exactly* when a second has started, enable
    * periodic update complete interrupts, (via ioctl) and then
    * immediately read /dev/rtc which will block until you get the IRQ.
    * Once the read clears, read the RTC time (again via ioctl). Easy.
    */
    -
    - if (rtc_is_updating() != 0)
    - while (jiffies - uip_watchdog < 2*HZ/100) {
    - barrier();
    - cpu_relax();
    - }
    + if (rtc_is_updating())
    + mdelay(20);

    /*
    * Only the values that we read from the RTC are set. We leave

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