linux-kernel - Re: [PATCH] kexec/crash: no crash update when kexec in progress

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Message-ID: <c6f30e31-69fe-4ece-b251-c49f1ab59a04@linux.ibm.com>
Date: Thu, 5 Sep 2024 14:07:58 +0530
From: Sourabh Jain <sourabhjain@...ux.ibm.com>
To: Baoquan He <bhe@...hat.com>
Cc: Michael Ellerman <mpe@...erman.id.au>,
        Hari Bathini <hbathini@...ux.ibm.com>, kexec@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, Sachin P Bappalige <sachinpb@...ux.vnet.ibm.com>
Subject: Re: [PATCH] kexec/crash: no crash update when kexec in progress

Hello Baoquan,

On 05/09/24 08:53, Baoquan He wrote:
> On 09/04/24 at 02:55pm, Sourabh Jain wrote:
>> Hello Baoquan,
>>
>> On 30/08/24 16:47, Baoquan He wrote:
>>> On 08/20/24 at 12:10pm, Sourabh Jain wrote:
>>>> Hello Baoquan,
>>>>
> ......snip...
>>>> 2. A patch to return early from the `crash_handle_hotplug_event()` function
>>>> if `kexec_in_progress` is
>>>>      set to True. This is essentially my original patch.
>>> There's a race gap between the kexec_in_progress checking and the
>>> setting it to true which Michael has mentioned.
>> The window where kernel is holding kexec_lock to do kexec boot
>> but kexec_in_progress is yet not set to True.
>>
>> If kernel needs to handle crash hotplug event, the function
>> crash_handle_hotplug_event()  will not get the kexec_lock and
>> error out by printing error message about not able to update
>> kdump image.
> But you wanted to avoid the erroring out if it's being in
> kernel_kexec().  Now you are seeing at least one the noising
> message, aren't you?

Yes, but it is very rare to encounter.

My comments on your updated code are inline below.

>
>> I think it should be fine. Given that lock is already taken for
>> kexec kernel boot.
>>
>> Am I missing something major?
>>
>>> That's why I think
>>> maybe checking kexec_in_progress after failing to retriving
>>> __kexec_lock is a little better, not very sure.
>> Try for kexec lock before kexec_in_progress check will not solve
>> the original problem this patch trying to solve.
>>
>> You proposed the below changes earlier:
>>
>> -	if (!kexec_trylock()) {
>> +	if (!kexec_trylock() && kexec_in_progress) {
>>   		pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
>>   		crash_hotplug_unlock();
> Ah, I meant as below, but wrote it mistakenly.
>
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 63cf89393c6e..e7c7aa761f46 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -504,7 +504,7 @@ int crash_check_hotplug_support(void)
>   
>   	crash_hotplug_lock();
>   	/* Obtain lock while reading crash information */
> -	if (!kexec_trylock()) {
> +	if (!kexec_trylock() && !kexec_in_progress) {
>   		pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
>   		crash_hotplug_unlock();
>   		return 0;
>
>
>>
>> Once the kexec_in_progress is set to True there is no way one can get
>> kexec_lock. So kexec_trylock() before kexec_in_progress is not helpful
>> for the problem I am trying to solve.
> With your patch, you could still get the error message if the race gap
> exist. With above change, you won't get it. Please correct me if I am
> wrong.

The above code will print an error message during the race gap. Here's why:

Let’s say the kexec lock is acquired in the kernel_kexec() function,
but kexec_in_progress is not yet set to True. In this scenario, the code 
will print
an error message.

There is another issue I see with the above code:

Consider that the system is on the kexec kernel boot path, and 
kexec_in_progress
is set to True. If crash_hotplug_unlock() is called, the kernel will not 
only update
the kdump image without acquiring the kexec lock, but it will also 
release the
kexec lock in the out label. I believe this is incorrect.

Please share your thoughts.

Thanks,
Sourabh Jain