Excerpts from Laurent Dufour's message of October 27, 2021 2:27 am:
> When handling the Watchdog interrupt, long processing should not be done
> while holding the __wd_smp_lock. This prevents the other CPUs to grab it
> and to process Watchdog timer interrupts. Furhtermore, this could lead to
> the following situation:
>
> CPU x detect lockup on CPU y and grab the __wd_smp_lock
> in watchdog_smp_panic()
> CPU y caught the watchdog interrupt and try to grab the __wd_smp_lock
> in soft_nmi_interrupt()
> CPU x wait for CPU y to catch the IPI for 1s in __smp_send_nmi_ipi()
CPU y should get the IPI here if it's a NMI IPI (which will be true for
>= POWER9 64s).
That said, not all platforms support it and the console lock problem
seems real, so okay.
> CPU x will timeout and so has spent 1s waiting while holding the
> __wd_smp_lock.
>
> A deadlock may also happen between the __wd_smp_lock and the console_owner
> 'lock' this way:
> CPU x grab the console_owner
> CPU y grab the __wd_smp_lock
> CPU x catch the watchdog timer interrupt and needs to grab __wd_smp_lock
> CPU y wants to print something and wait for console_owner
> -> deadlock
>
> Doing all the long processing without holding the _wd_smp_lock prevents
> these situations.
The intention was to avoid logs getting garbled e.g., if multiple
different CPUs fire at once.
I wonder if instead we could deal with that by protecting the IPI
sending and printing stuff with a trylock, and if you don't get the
trylock then just return, and you'll come back with the next timer
interrupt.
Thanks,
Nick
>
> Signed-off-by: Laurent Dufour <lduf...@linux.ibm.com>
> ---
> arch/powerpc/kernel/watchdog.c | 31 +++++++++++++++++--------------
> 1 file changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
> index f9ea0e5357f9..bc7411327066 100644
> --- a/arch/powerpc/kernel/watchdog.c
> +++ b/arch/powerpc/kernel/watchdog.c
> @@ -149,6 +149,8 @@ static void set_cpu_stuck(int cpu, u64 tb)
>
> static void watchdog_smp_panic(int cpu, u64 tb)
> {
> + cpumask_t cpus_pending_copy;
> + u64 last_reset_tb_copy;
> unsigned long flags;
> int c;
>
> @@ -161,29 +163,32 @@ static void watchdog_smp_panic(int cpu, u64 tb)
> if (cpumask_weight(&wd_smp_cpus_pending) == 0)
> goto out;
>
> + cpumask_copy(&cpus_pending_copy, &wd_smp_cpus_pending);
> + last_reset_tb_copy = wd_smp_last_reset_tb;
> +
> + /* Take the stuck CPUs out of the watch group */
> + set_cpumask_stuck(&wd_smp_cpus_pending, tb);
> +
> + wd_smp_unlock(&flags);
> +
> pr_emerg("CPU %d detected hard LOCKUP on other CPUs %*pbl\n",
> - cpu, cpumask_pr_args(&wd_smp_cpus_pending));
> + cpu, cpumask_pr_args(&cpus_pending_copy));
> pr_emerg("CPU %d TB:%lld, last SMP heartbeat TB:%lld (%lldms ago)\n",
> - cpu, tb, wd_smp_last_reset_tb,
> - tb_to_ns(tb - wd_smp_last_reset_tb) / 1000000);
> + cpu, tb, last_reset_tb_copy,
> + tb_to_ns(tb - last_reset_tb_copy) / 1000000);
>
> if (!sysctl_hardlockup_all_cpu_backtrace) {
> /*
> * Try to trigger the stuck CPUs, unless we are going to
> * get a backtrace on all of them anyway.
> */
> - for_each_cpu(c, &wd_smp_cpus_pending) {
> + for_each_cpu(c, &cpus_pending_copy) {
> if (c == cpu)
> continue;
> smp_send_nmi_ipi(c, wd_lockup_ipi, 1000000);
> }
> }
>
> - /* Take the stuck CPUs out of the watch group */
> - set_cpumask_stuck(&wd_smp_cpus_pending, tb);
> -
> - wd_smp_unlock(&flags);
> -
> if (sysctl_hardlockup_all_cpu_backtrace)
> trigger_allbutself_cpu_backtrace();
>
> @@ -204,6 +209,8 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
> unsigned long flags;
>
> wd_smp_lock(&flags);
> + cpumask_clear_cpu(cpu, &wd_smp_cpus_stuck);
> + wd_smp_unlock(&flags);
>
> pr_emerg("CPU %d became unstuck TB:%lld\n",
> cpu, tb);
> @@ -212,9 +219,6 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
> show_regs(regs);
> else
> dump_stack();
> -
> - cpumask_clear_cpu(cpu, &wd_smp_cpus_stuck);
> - wd_smp_unlock(&flags);
> }
> return;
> }
> @@ -267,6 +271,7 @@ DEFINE_INTERRUPT_HANDLER_NMI(soft_nmi_interrupt)
> return 0;
> }
> set_cpu_stuck(cpu, tb);
> + wd_smp_unlock(&flags);
>
> pr_emerg("CPU %d self-detected hard LOCKUP @ %pS\n",
> cpu, (void *)regs->nip);
> @@ -277,8 +282,6 @@ DEFINE_INTERRUPT_HANDLER_NMI(soft_nmi_interrupt)
> print_irqtrace_events(current);
> show_regs(regs);
>
> - wd_smp_unlock(&flags);
> -
> if (sysctl_hardlockup_all_cpu_backtrace)
> trigger_allbutself_cpu_backtrace();
>
> --
> 2.33.1
>
>
