On 09/05/24 at 02:07pm, Sourabh Jain wrote:

Hello Baoquan,

On 05/09/24 08:53, Baoquan He wrote:

quoted

On 09/04/24 at 02:55pm, Sourabh Jain wrote:

quoted

Hello Baoquan,

On 30/08/24 16:47, Baoquan He wrote:

quoted

On 08/20/24 at 12:10pm, Sourabh Jain wrote:

quoted

Hello Baoquan,

......snip...

quoted

quoted

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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.

quoted

quoted

I think it should be fine. Given that lock is already taken for
kexec kernel boot.

Am I missing something major?

quoted

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;

quoted

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.

How about this?

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 63cf89393c6e..8ba7b1da0ded 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c

@@ -505,7 +505,8 @@ int crash_check_hotplug_support(void)
 	crash_hotplug_lock();
 	/* Obtain lock while reading crash information */
 	if (!kexec_trylock()) {
-		pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
+		if (!kexec_in_progress)
+			pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
 		crash_hotplug_unlock();
 		return 0;
 	}

@@ -540,7 +541,8 @@ 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()) {
-		pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
+		if (!kexec_in_progress)
+			pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
 		crash_hotplug_unlock();
 		return;
 	}