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    SubjectRe: [stable] [patch 31/32] x86_64: fix boot hang due to nmi watchdog init code
    * Chris Wright (chrisw@sous-sol.org) wrote:
    > -stable review patch. If anyone has any objections, please let us know.
    > ------------------

    replaced with upstream version, which is slightly changed by Andi.
    --

    From: Ravikiran G Thirumalai <kiran@scalex86.org>

    2.6.19 stopped booting (or booted based on build/config) on our x86_64
    systems due to a bug introduced in 2.6.19. check_nmi_watchdog schedules an
    IPI on all cpus to busy wait on a flag, but fails to set the busywait
    flag if NMI functionality is disabled. This causes the secondary cpus
    to spin in an endless loop, causing the kernel bootup to hang.
    Depending upon the build, the busywait flag got overwritten (stack variable)
    and caused the kernel to bootup on certain builds. Following patch fixes
    the bug by setting the busywait flag before returning from check_nmi_watchdog.
    I guess using a stack variable is not good here as the calling function could
    potentially return while the busy wait loop is still spinning on the flag.

    AK: I redid the patch significantly to be cleaner

    Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
    Signed-off-by: Shai Fultheim <shai@scalex86.org>
    Signed-off-by: Andi Kleen <ak@suse.de>
    Signed-off-by: Chris Wright <chrisw@sous-sol.org>
    ---
    arch/i386/kernel/nmi.c | 8 ++++----
    arch/x86_64/kernel/nmi.c | 9 +++++----
    2 files changed, 9 insertions(+), 8 deletions(-)

    --- linux-2.6.19.orig/arch/i386/kernel/nmi.c
    +++ linux-2.6.19/arch/i386/kernel/nmi.c
    @@ -192,6 +192,8 @@ static __cpuinit inline int nmi_known_cp
    return 0;
    }

    +static int endflag __initdata = 0;
    +
    #ifdef CONFIG_SMP
    /* The performance counters used by NMI_LOCAL_APIC don't trigger when
    * the CPU is idle. To make sure the NMI watchdog really ticks on all
    @@ -199,7 +201,6 @@ static __cpuinit inline int nmi_known_cp
    */
    static __init void nmi_cpu_busy(void *data)
    {
    - volatile int *endflag = data;
    local_irq_enable_in_hardirq();
    /* Intentionally don't use cpu_relax here. This is
    to make sure that the performance counter really ticks,
    @@ -207,14 +208,13 @@ static __init void nmi_cpu_busy(void *da
    pause instruction. On a real HT machine this is fine because
    all other CPUs are busy with "useless" delay loops and don't
    care if they get somewhat less cycles. */
    - while (*endflag == 0)
    - barrier();
    + while (endflag == 0)
    + mb();
    }
    #endif

    static int __init check_nmi_watchdog(void)
    {
    - volatile int endflag = 0;
    unsigned int *prev_nmi_count;
    int cpu;

    --- linux-2.6.19.orig/arch/x86_64/kernel/nmi.c
    +++ linux-2.6.19/arch/x86_64/kernel/nmi.c
    @@ -190,6 +190,8 @@ void nmi_watchdog_default(void)
    nmi_watchdog = NMI_IO_APIC;
    }

    +static int endflag __initdata = 0;
    +
    #ifdef CONFIG_SMP
    /* The performance counters used by NMI_LOCAL_APIC don't trigger when
    * the CPU is idle. To make sure the NMI watchdog really ticks on all
    @@ -197,7 +199,6 @@ void nmi_watchdog_default(void)
    */
    static __init void nmi_cpu_busy(void *data)
    {
    - volatile int *endflag = data;
    local_irq_enable_in_hardirq();
    /* Intentionally don't use cpu_relax here. This is
    to make sure that the performance counter really ticks,
    @@ -205,14 +206,13 @@ static __init void nmi_cpu_busy(void *da
    pause instruction. On a real HT machine this is fine because
    all other CPUs are busy with "useless" delay loops and don't
    care if they get somewhat less cycles. */
    - while (*endflag == 0)
    - barrier();
    + while (endflag == 0)
    + mb();
    }
    #endif

    int __init check_nmi_watchdog (void)
    {
    - volatile int endflag = 0;
    int *counts;
    int cpu;

    @@ -253,6 +253,7 @@ int __init check_nmi_watchdog (void)
    if (!atomic_read(&nmi_active)) {
    kfree(counts);
    atomic_set(&nmi_active, -1);
    + endflag = 1;
    return -1;
    }
    endflag = 1;
    -
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