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    SubjectRe: [PATCH] clocksource: disable irq when holding watchdog_lock.
    On Fri, Oct 13, 2023 at 7:51 AM Tetsuo Handa
    <penguin-kernel@i-love.sakura.ne.jp> wrote:
    >
    > Lockdep found that spin_lock(&watchdog_lock) from call_timer_fn()
    > is not safe. Use spin_lock_irqsave(&watchdog_lock, flags) instead.
    >
    > [ 0.378387] TSC synchronization [CPU#0 -> CPU#1]:
    > [ 0.378387] Measured 55060 cycles TSC warp between CPUs, turning off TSC clock.
    > [ 0.378387] tsc: Marking TSC unstable due to check_tsc_sync_source failed
    > [ 0.926101]
    > [ 0.926387] ================================
    > [ 0.926387] WARNING: inconsistent lock state
    > [ 0.926387] 6.6.0-rc5-00192-g10a6e5feccb8 #2 Not tainted
    > [ 0.926387] --------------------------------
    > [ 0.926387] inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
    > [ 0.926387] swapper/0/0 [HC0[0]:SC1[1]:HE1:SE0] takes:
    > [ 0.926387] ffffffff8cfa1c78 (watchdog_lock){?.-.}-{2:2}, at: clocksource_watchdog+0x23/0x5a0
    > [ 0.926387] {IN-HARDIRQ-W} state was registered at:
    > [ 0.926387] lock_acquire+0xc1/0x2c0
    > [ 0.926387] _raw_spin_lock_irqsave+0x3f/0x60
    > [ 0.926387] clocksource_mark_unstable+0x1b/0x90
    > [ 0.926387] mark_tsc_unstable+0x41/0x50
    > [ 0.926387] check_tsc_sync_source+0x14f/0x180
    > [ 0.926387] __flush_smp_call_function_queue+0x16f/0x560
    > [ 0.926387] __sysvec_call_function_single+0x36/0x110
    > [ 0.926387] sysvec_call_function_single+0x69/0x90
    > [ 0.926387] asm_sysvec_call_function_single+0x1a/0x20
    > [ 0.926387] default_idle+0xf/0x20
    > [ 0.926387] default_idle_call+0x7f/0x180
    > [ 0.926387] do_idle+0x1e1/0x220
    > [ 0.926387] cpu_startup_entry+0x2a/0x30
    > [ 0.926387] rest_init+0xf4/0x190
    > [ 0.926387] arch_call_rest_init+0xe/0x30
    > [ 0.926387] start_kernel+0x763/0x910
    > [ 0.926387] x86_64_start_reservations+0x18/0x30
    > [ 0.926387] x86_64_start_kernel+0xca/0xe0
    > [ 0.926387] secondary_startup_64_no_verify+0x178/0x17b
    > [ 0.926387] irq event stamp: 138774
    > [ 0.926387] hardirqs last enabled at (138774): [<ffffffff8bb99968>] _raw_spin_unlock_irq+0x28/0x50
    > [ 0.926387] hardirqs last disabled at (138773): [<ffffffff8bb99687>] _raw_spin_lock_irq+0x47/0x50
    > [ 0.926387] softirqs last enabled at (138216): [<ffffffff8b0904ff>] irq_exit_rcu+0x7f/0xa0
    > [ 0.926387] softirqs last disabled at (138267): [<ffffffff8b0904ff>] irq_exit_rcu+0x7f/0xa0
    > [ 0.926387]
    > [ 0.926387] other info that might help us debug this:
    > [ 0.926387] Possible unsafe locking scenario:
    > [ 0.926387]
    > [ 0.926387] CPU0
    > [ 0.926387] ----
    > [ 0.926387] lock(watchdog_lock);
    > [ 0.926387] <Interrupt>
    > [ 0.926387] lock(watchdog_lock);
    > [ 0.926387]
    > [ 0.926387] *** DEADLOCK ***
    ...
    >
    > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
    > index c108ed8a9804..4e8fc0a5ca9d 100644
    > --- a/kernel/time/clocksource.c
    > +++ b/kernel/time/clocksource.c
    > @@ -405,8 +405,9 @@ static void clocksource_watchdog(struct timer_list *unused)
    > enum wd_read_status read_ret;
    > unsigned long extra_wait = 0;
    > u32 md;
    > + unsigned long flags;
    >
    > - spin_lock(&watchdog_lock);
    > + spin_lock_irqsave(&watchdog_lock, flags);
    > if (!watchdog_running)
    > goto out;
    >
    > @@ -554,7 +555,7 @@ static void clocksource_watchdog(struct timer_list *unused)
    > add_timer_on(&watchdog_timer, next_cpu);
    > }
    > out:
    > - spin_unlock(&watchdog_lock);
    > + spin_unlock_irqrestore(&watchdog_lock, flags);
    > }
    >
    > static inline void clocksource_start_watchdog(void)
    > --

    Hey! Thank you for the report and the patch!

    At first glance this looks sane to me. The only thing that gives me
    pause is that I'm a little shocked this hasn't been caught earlier,
    and I don't right off see recent changes that would have caused this
    to trip. But Thomas may have more insight.

    Tentatively:
    Acked-by: John Stultz <jstultz@google.com>

    thanks
    -john

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