[PATCH v2 1/9] panic: Introduce helper functions for panic state

[Jinchao Wang <wangjinchao600@gmail.com>]

This patch introduces four new helper functions to abstract the
management of the panic_cpu variable. These functions will be used in
subsequent patches to refactor existing code.

The direct use of panic_cpu can be error-prone and ambiguous, as it
requires manual checks to determine which CPU is handling the panic.
The new helpers clarify intent:

panic_try_start():
Atomically sets the current CPU as the panicking CPU.

panic_reset():
Reset panic_cpu to PANIC_CPU_INVALID.

panic_in_progress():
Checks if a panic has been triggered.

panic_on_this_cpu():
Returns true if the current CPU is the panic originator.

panic_on_other_cpu():
Returns true if a panic is on another CPU.

This change lays the groundwork for improved code readability
and robustness in the panic handling subsystem.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 include/linux/panic.h  |  6 +++++
 kernel/panic.c         | 53 ++++++++++++++++++++++++++++++++++++++++++
 kernel/printk/printk.c |  5 ----
 3 files changed, 59 insertions(+), 5 deletions(-)

diff --git a/include/linux/panic.h b/include/linux/panic.h
index 7be742628c25..6f972a66c13e 100644
--- a/include/linux/panic.h
+++ b/include/linux/panic.h
@@ -43,6 +43,12 @@ void abort(void);
 extern atomic_t panic_cpu;
 #define PANIC_CPU_INVALID	-1
 
+bool panic_try_start(void);
+void panic_reset(void);
+bool panic_in_progress(void);
+bool panic_on_this_cpu(void);
+bool panic_on_other_cpu(void);
+
 /*
  * Only to be used by arch init code. If the user over-wrote the default
  * CONFIG_PANIC_TIMEOUT, honor it.
diff --git a/kernel/panic.c b/kernel/panic.c
index 72fcbb5a071b..eacb0c972110 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -294,6 +294,59 @@ void __weak crash_smp_send_stop(void)
 
 atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
 
+bool panic_try_start(void)
+{
+	int old_cpu, this_cpu;
+
+	/*
+	 * Only one CPU is allowed to execute the crash_kexec() code as with
+	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
+	 * may stop each other.  To exclude them, we use panic_cpu here too.
+	 */
+	old_cpu = PANIC_CPU_INVALID;
+	this_cpu = raw_smp_processor_id();
+
+	return atomic_try_cmpxchg(&panic_cpu, &old_cpu, this_cpu);
+}
+EXPORT_SYMBOL(panic_try_start);
+
+void panic_reset(void)
+{
+	atomic_set(&panic_cpu, PANIC_CPU_INVALID);
+}
+EXPORT_SYMBOL(panic_reset);
+
+bool panic_in_progress(void)
+{
+	return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
+}
+EXPORT_SYMBOL(panic_in_progress);
+
+/* Return true if a panic is in progress on the current CPU. */
+bool panic_on_this_cpu(void)
+{
+	/*
+	 * We can use raw_smp_processor_id() here because it is impossible for
+	 * the task to be migrated to the panic_cpu, or away from it. If
+	 * panic_cpu has already been set, and we're not currently executing on
+	 * that CPU, then we never will be.
+	 */
+	return unlikely(atomic_read(&panic_cpu) == raw_smp_processor_id());
+}
+EXPORT_SYMBOL(panic_on_this_cpu);
+
+/*
+ * Return true if a panic is in progress on a remote CPU.
+ *
+ * On true, the local CPU should immediately release any printing resources
+ * that may be needed by the panic CPU.
+ */
+bool panic_on_other_cpu(void)
+{
+	return (panic_in_progress() && !this_cpu_in_panic());
+}
+EXPORT_SYMBOL(panic_on_other_cpu);
+
 /*
  * A variant of panic() called from NMI context. We return if we've already
  * panicked on this CPU. If another CPU already panicked, loop in
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0efbcdda9aab..5fe35f377b79 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -345,11 +345,6 @@ static void __up_console_sem(unsigned long ip)
 }
 #define up_console_sem() __up_console_sem(_RET_IP_)
 
-static bool panic_in_progress(void)
-{
-	return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
-}
-
 /* Return true if a panic is in progress on the current CPU. */
 bool this_cpu_in_panic(void)
 {
-- 
2.43.0