答复: 答复: 答复: 答复: 答复: 答复: [PATCH] kdb: Fix the deadlock issue in KDB debugging.

Liuye
2024-03-15T10:00:20Z

>On Thu, Mar 14, 2024 at 07:06:22AM +0000, Liuye wrote:
>> >On Wed, Mar 13, 2024 at 01:22:17AM +0000, Liuye wrote:
>> >> >On Tue, Mar 12, 2024 at 10:04:54AM +0000, Liuye wrote:
>> >> >> >On Tue, Mar 12, 2024 at 08:37:11AM +0000, Liuye wrote:
>> >> >> >> I know that you said schedule_work is not NMI save, which is
>> >> >> >> the first issue. Perhaps it can be fixed using
>> >> >> >> irq_work_queue. But even if irq_work_queue is used to
>> >> >> >> implement it, there will still be a deadlock problem because
>> >> >> >> slave cpu1 still has not released the running queue lock of
>> >> >> >> master CPU0.
>> >> >> >
>> >> >> >This doesn't sound right to me. Why do you think CPU1 won't
>> >> >> >release the run queue lock?
>> >> >>
>> >> >> In this example, CPU1 is waiting for CPU0 to release
>> >> >> dbg_slave_lock.
>> >> >
>> >> >That shouldn't be a problem. CPU0 will have released that lock by
>> >> >the time the irq work is dispatched.
>> >>
>> >> Release dbg_slave_lock in CPU0. Before that, shcedule_work needs to
>> >> be handled, and we are back to the previous issue.
>> >
>> > Sorry but I still don't understand what problem you think can happen
>> > here. What is wrong with calling schedule_work() from the IRQ work
>> > handler?
>> >
>> > Both irq_work_queue() and schedule_work() are calls to queue
>> > deferred work. It does not matter when the work is queued (providing
>> > we are lock safe). What matters is when the work is actually executed.
>> >
>> > Please can you describe the problem you think exists based on when
>> > the work is executed.
>>
>> CPU0 enters the KDB process when processing serial port interrupts and
>> triggers an IPI (NMI) to other CPUs. After entering a stable state,
>> CPU0 is in interrupt context, while other CPUs are in NMI context.
>> Before other CPUs enter NMI context, there is a chance to obtain the
>> running queue of CPU0.
>
>Focusing on the run queue locks in this analysis is a mistake. Before the other CPUs enter NMI context there is a chance for them to obtain
>*any* locks, including the timer wheel locks.

Yes, you are right. I did not consider it comprehensively.

>> At this time, when CPU0 is processing kgdboc_restore_input, calling
>> schedule_work, need_more_worker here determines the chance to wake up
>> processes on system_wq.
>>
>> This will cause CPU0 to acquire the running queue lock of this core,
>> which is held by other CPUs. but other CPUs are still in NMI context
>> and have not exited because waiting for CPU0 to release the
>> dbg_slave_lock after schedule_work.
>>
>> After thinking about it, the problem is not whether schedule_work is
>> NMI safe, but that processes on system_wq should not be awakened
>> immediately when schedule_work is called.
>
>I disagree with this conclusion.
>
>The problem *is* that schedue_work() is not NMI-safe.
>
>You cannot solve an NMI safety problem by replacing schedule_work() with another function that is also not NMI-safe. That simply changes the locks that need to be taken to provoke a deadlock.
>
>
>> I replaced schedule_work with schedule_delayed_work, and this solved
>> my problem.
>
>This may stop some specific reproduction from taking place but it does not fix the potential deadlock.
>
>I still believe that using irq_work is the only way to solve this properly. Please try the following change:

I tried the following patch and it also resolved the issue.
Thank you for your guidance and suggestions. I will organize this issue and resend a patch.

>diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c index 7ce7bb1640054..161b25ecc5e15 100644
>--- a/drivers/tty/serial/kgdboc.c
>+++ b/drivers/tty/serial/kgdboc.c
>@@ -22,6 +22,7 @@
> #include <linux/module.h>
> #include <linux/platform_device.h>
> #include <linux/serial_core.h>
>+#include <linux/irq_work.h>
>
> #define MAX_CONFIG_LEN 40
>
>@@ -99,10 +100,17 @@ static void kgdboc_restore_input_helper(struct work_struct *dummy)
>
> static DECLARE_WORK(kgdboc_restore_input_work, kgdboc_restore_input_helper);
>
>+static void kgdboc_queue_restore_input_helper(struct irq_work *unused)
>+{
>+ schedule_work(&kgdboc_restore_input_work);
>+}
>+
>+static DEFINE_IRQ_WORK(kgdboc_restore_input_irq_work,
>+kgdboc_queue_restore_input_helper);
>+
> static void kgdboc_restore_input(void)
> {
> if (likely(system_state == SYSTEM_RUNNING))
>- schedule_work(&kgdboc_restore_input_work);
>+ irq_work_queue(&kgdboc_restore_input_irq_work);
> }
>
> static int kgdboc_register_kbd(char **cptr) @@ -133,6 +141,7 @@ static void kgdboc_unregister_kbd(void)
> i--;
> }
> }
>+ irq_work_sync(&kgdboc_restore_input_irq_work);
> flush_work(&kgdboc_restore_input_work);
> }
> #else /* ! CONFIG_KDB_KEYBOARD */