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    SubjectRe: fix BUG: using smp_processor_id() in touch_nmi_watchdog and touch_softlockup_watchdog
    On Mon, Aug 16, 2010 at 10:22:50AM +0200, Peter Zijlstra wrote:
    > On Fri, 2010-08-13 at 13:21 +0300, Sergey Senozhatsky wrote:
    >
    > > [ 67.703556] BUG: using smp_processor_id() in preemptible [00000000] code: s2disk/5139
    > > [ 67.703563] caller is touch_nmi_watchdog+0x15/0x2c
    > > [ 67.703566] Pid: 5139, comm: s2disk Not tainted 2.6.36-rc0-git12-07921-g60bf26a-dirty #116
    > > [ 67.703568] Call Trace:
    > > [ 67.703575] [<ffffffff811f6bf1>] debug_smp_processor_id+0xc9/0xe4
    > > [ 67.703578] [<ffffffff81092766>] touch_nmi_watchdog+0x15/0x2c
    > > [ 67.703584] [<ffffffff81222950>] acpi_os_stall+0x34/0x40
    > > [ 67.703589] [<ffffffff812398d2>] acpi_ex_system_do_stall+0x34/0x38
    >
    > Which could mean two things, either ACPI got funny on us, or Don's new
    > watchdog stuff has a hole in it.

    it could. :-)

    >
    >
    > > ---
    > >
    > > diff --git a/kernel/watchdog.c b/kernel/watchdog.c
    > > index 613bc1f..8822f1e 100644
    > > --- a/kernel/watchdog.c
    > > +++ b/kernel/watchdog.c
    > > @@ -116,13 +116,14 @@ static unsigned long get_sample_period(void)
    > > static void __touch_watchdog(void)
    > > {
    > > int this_cpu = smp_processor_id();
    > > -
    > > - __get_cpu_var(watchdog_touch_ts) = get_timestamp(this_cpu);
    > > + per_cpu(watchdog_touch_ts, this_cpu) = get_timestamp(this_cpu);
    > > }
    >
    > That change seems sensible enough..

    ok.

    >
    > > void touch_softlockup_watchdog(void)
    > > {
    > > - __get_cpu_var(watchdog_touch_ts) = 0;
    > > + int this_cpu = get_cpu();
    > > + per_cpu(watchdog_touch_ts, this_cpu) = 0;
    > > + put_cpu();
    > > }
    > > EXPORT_SYMBOL(touch_softlockup_watchdog);
    > >
    > > @@ -142,7 +143,9 @@ void touch_all_softlockup_watchdogs(void)
    > > #ifdef CONFIG_HARDLOCKUP_DETECTOR
    > > void touch_nmi_watchdog(void)
    > > {
    > > - __get_cpu_var(watchdog_nmi_touch) = true;
    > > + int this_cpu = get_cpu();
    > > + per_cpu(watchdog_nmi_touch, this_cpu) = true;
    > > + put_cpu();
    > > touch_softlockup_watchdog();
    > > }
    > > EXPORT_SYMBOL(touch_nmi_watchdog);
    >
    > These other two really are about assumptions we make on the call sites,
    > which at the very least are violated by ACPI.
    >
    > Don/Ingo, remember if we require touch_*_watchdog callers to have
    > preemption disabled? Or is the proposed patch sensible?

    I don't recall any requirement to have preemption disabled when using
    those functions. It seems sensible to put it in the
    touch_{softlockup|nmi}_watchdog code.

    I assume the reason for having preemption disabled when using
    smp_processor_id() is that the code could migrate to another cpu when
    rescheduled?

    I don't see a problem with the patch, but my low level understanding of
    the __get_cpu_var vs. per_cpu isn't very strong.

    Cheers,
    Don

    >


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