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

[Sourabh Jain]

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@xxxxxxxxxxxxx> 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