[PATCH v2 3/6] panic: Allow postponing pre-kdump notifiers

[Zack Rusin]

Provide an escape hatch for debugging kdump failures that may involve
pre-kdump callbacks. Add panic_pre_kdump_postpone to skip their early
invocation in crash core without consuming the once-only guard.

When enabled, the chain runs before kdump only when
crash_kexec_post_notifiers is also enabled. Leave the late panic call
unchanged so callbacks remain eligible if that path is reached.
Skipping the callbacks may prevent hypervisor crash handling.

Suggested-by: Guilherme G. Piccoli <gpiccoli at igalia.com>
Link: https://lore.kernel.org/r/bc0bdd14-5e6a-0676-cd04-14fb0bb93cd5@igalia.com
Signed-off-by: Zack Rusin <zack.rusin at broadcom.com>
---
New in v2: optional postponement policy suggested by Guilherme.

 Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
 include/linux/panic_notifier.h                  |  1 +
 kernel/crash_core.c                             |  3 ++-
 kernel/panic.c                                  |  2 ++
 4 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 33cd30996e47..3d722df30838 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4904,6 +4904,20 @@ Kernel parameters
 			This option should only be used for systems with the above
 			constraints as it might cause the panic operation to be less reliable.
 
+	panic_pre_kdump_postpone=
+			[KNL] Defer pre-kdump notifiers to the late panic path.
+			They run before kdump only when
+			crash_kexec_post_notifiers is also enabled. If panic
+			reaches the late path without entering kdump, they
+			can still run there.
+			Intended for debugging kexec/crash-dump failures.
+			Skipping these callbacks may prevent correct crash
+			handling by the hypervisor.
+			Format: <bool>
+			Default: 0
+			Writable at runtime via
+			/sys/module/kernel/parameters/panic_pre_kdump_postpone.
+
 	panic_print=	Bitmask for printing system info when panic happens.
 			User can chose combination of the following bits:
 
diff --git a/include/linux/panic_notifier.h b/include/linux/panic_notifier.h
index 3ad390764a41..655ee5c77181 100644
--- a/include/linux/panic_notifier.h
+++ b/include/linux/panic_notifier.h
@@ -11,5 +11,6 @@ extern struct atomic_notifier_head panic_pre_kdump_list;
 void panic_notify_pre_kdump(char *msg);
 
 extern bool crash_kexec_post_notifiers;
+extern bool panic_pre_kdump_postpone;
 
 #endif	/* _LINUX_PANIC_NOTIFIERS_H */
diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 5d9fe9e00f39..d6b2293dd0fe 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -140,7 +140,8 @@ void __noclone __crash_kexec(struct pt_regs *regs)
 			struct pt_regs fixed_regs;
 
 			crash_setup_regs(&fixed_regs, regs);
-			panic_notify_pre_kdump(NULL);
+			if (!READ_ONCE(panic_pre_kdump_postpone))
+				panic_notify_pre_kdump(NULL);
 			crash_save_vmcoreinfo();
 			machine_crash_shutdown(&fixed_regs);
 			crash_cma_clear_pending_dma();
diff --git a/kernel/panic.c b/kernel/panic.c
index d78e90abb192..65d86594e038 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -62,6 +62,7 @@ static int pause_on_oops;
 static int pause_on_oops_flag;
 static DEFINE_SPINLOCK(pause_on_oops_lock);
 bool crash_kexec_post_notifiers;
+bool panic_pre_kdump_postpone;
 int panic_on_warn __read_mostly;
 unsigned long panic_on_taint;
 bool panic_on_taint_nousertaint = false;
@@ -1243,6 +1244,7 @@ core_param(panic, panic_timeout, int, 0644);
 core_param(pause_on_oops, pause_on_oops, int, 0644);
 core_param(panic_on_warn, panic_on_warn, int, 0644);
 core_param(crash_kexec_post_notifiers, crash_kexec_post_notifiers, bool, 0644);
+core_param(panic_pre_kdump_postpone, panic_pre_kdump_postpone, bool, 0644);
 core_param(panic_console_replay, panic_console_replay, bool, 0644);
 
 static int panic_print_set(const char *val, const struct kernel_param *kp)