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

[Sourabh Jain]

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