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

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Message-ID: <1e4a8e18-cda9-45f5-a842-8ffcd725efc9@linux.ibm.com>
Date: Tue, 20 Aug 2024 12:10:27 +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 19/08/24 11:45, Baoquan He wrote:
> On 08/19/24 at 09:45am, Sourabh Jain wrote:
>> Hello Michael and Boaquan
>>
>> On 01/08/24 12:21, Sourabh Jain wrote:
>>> Hello Michael,
>>>
>>> On 01/08/24 08:04, Michael Ellerman wrote:
>>>> Sourabh Jain <sourabhjain@...ux.ibm.com> writes:
>>>>> The following errors are observed when kexec is done with SMT=off on
>>>>> powerpc.
>>>>>
>>>>> [  358.458385] Removing IBM Power 842 compression device
>>>>> [  374.795734] kexec_core: Starting new kernel
>>>>> [  374.795748] kexec: Waking offline cpu 1.
>>>>> [  374.875695] crash hp: kexec_trylock() failed, elfcorehdr may
>>>>> be inaccurate
>>>>> [  374.935833] kexec: Waking offline cpu 2.
>>>>> [  375.015664] crash hp: kexec_trylock() failed, elfcorehdr may
>>>>> be inaccurate
>>>>> snip..
>>>>> [  375.515823] kexec: Waking offline cpu 6.
>>>>> [  375.635667] crash hp: kexec_trylock() failed, elfcorehdr may
>>>>> be inaccurate
>>>>> [  375.695836] kexec: Waking offline cpu 7.
>>>> Are they actually errors though? Do they block the actual kexec from
>>>> happening? Or are they just warnings in dmesg?
>>> The kexec kernel boots fine.
>>>
>>> This warning appears regardless of whether the kdump kernel is loaded.
>>>
>>> However, when the kdump kernel is loaded, we will not be able to update
>>> the kdump image (FDT).
>>> I think this should be fine given that kexec is in progress.
>>>
>>> Please let me know your opinion.
>>>
>>>> Because the fix looks like it could be racy.
>>> It seems like it is racy, but given that kexec takes the lock first and
>>> then
>>> brings the CPU up, which triggers the kdump image, which always fails to
>>> update the kdump image because it could not take the same lock.
>>>
>>> Note: the kexec lock is not released unless kexec boot fails.
>> Any comments or suggestions on this fix?
> Is this a little better?
>
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 63cf89393c6e..0355ffb712f4 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;
> @@ -539,7 +539,7 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu,
>   
>   	crash_hotplug_lock();
>   	/* Obtain lock while changing 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;

Ideally, when `kexec_in_progress` is True, there should be no way to 
acquire the kexec lock.
Therefore, calling `kexec_trylock()` before checking `kexec_in_progress` 
is not helpful.
The kernel will print the error message "kexec_trylock() failed, 
elfcorehdr may be inaccurate."
So, with the above changes, the original problem remains unsolved.

However, after closely inspecting the 
`kernel/kexec_core.c:kernel_kexec()` function, I discovered
an exceptional case where my patch needs an update. The issue arises 
when the system returns
from the `machine_kexec()` function, which indicates that kexec has failed.

In this scenario, the kexec lock is released, but `kexec_in_progress` 
remains True.

I am unsure why `kexec_in_progress` is NOT set to False when kexec 
fails. Was this by design,
or was it an oversight because returning from the `machine_kexec()` 
function is highly unlikely?

Here is my proposal to address the original problem along with the 
exceptional case I described
above.

Let's implement two patches:

1. A patch that sets `kexec_in_progress` to False if the system returns 
from `machine_kexec()` before
    unlocking the kexec lock in the `kernel_kexec()` function.

    ```
    diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
    index c0caa14880c3..b41277183455 100644
    --- a/kernel/kexec_core.c
    +++ b/kernel/kexec_core.c
    @@ -1069,6 +1069,7 @@ int kernel_kexec(void)
    #endif

     Unlock:
    +      kexec_in_progress = false;
            kexec_unlock();
            return error;
     ```

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.

Please share your comments on the new approach.

Thank you for review.

- Sourabh Jain