[PATCH] Crash: add lock to serialize crash hotplug handling

[Baoquan He <bhe@redhat.com>]

Eric reported that handling corresponding crash hotplug event can be
failed easily when many momery hotplug event are notified in a short period.
They failed because failing to take __kexec_lock.

=======
[   78.714569] Fallback order for Node 0: 0
[   78.714575] Built 1 zonelists, mobility grouping on.  Total pages: 1817886
[   78.717133] Policy zone: Normal
[   78.724423] crash hp: kexec_trylock() failed, elfcorehdr may be inaccurate
[   78.727207] crash hp: kexec_trylock() failed, elfcorehdr may be inaccurate
[   80.056643] PEFILE: Unsigned PE binary
=======

The memory hotplug events are notified very quickly and very many,
while the handling of crash hotplug is much slower relatively. So the
atomic variable __kexec_lock and kexec_trylock() can't guarantee the
serialization of crash hotplug handling.

Here, add a new mutex lock __crash_hotplug_lock to serialize crash
hotplug handling specifically. This doesn't impact the usage of
__kexec_lock.

Signed-off-by: Baoquan He <bhe@redhat.com>
---
 kernel/crash_core.c     |  3 +++
 kernel/kexec_core.c     |  1 +
 kernel/kexec_internal.h | 11 +++++++++++
 3 files changed, 15 insertions(+)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 03a7932cde0a..e8851724a530 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -783,9 +783,11 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
 {
 	struct kimage *image;
 
+	crash_hotplug_lock();
 	/* Obtain lock while changing crash information */
 	if (!kexec_trylock()) {
 		pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
+		crash_hotplug_unlock();
 		return;
 	}
 
@@ -852,6 +854,7 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
 out:
 	/* Release lock now that update complete */
 	kexec_unlock();
+	crash_hotplug_unlock();
 }
 
 static int crash_memhp_notifier(struct notifier_block *nb, unsigned long val, void *v)
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 9dc728982d79..b95a73f35d9a 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -48,6 +48,7 @@
 #include "kexec_internal.h"
 
 atomic_t __kexec_lock = ATOMIC_INIT(0);
+DEFINE_MUTEX(__crash_hotplug_lock);
 
 /* Flag to indicate we are going to kexec a new kernel */
 bool kexec_in_progress = false;
diff --git a/kernel/kexec_internal.h b/kernel/kexec_internal.h
index 74da1409cd14..1db31625ef20 100644
--- a/kernel/kexec_internal.h
+++ b/kernel/kexec_internal.h
@@ -28,6 +28,17 @@ static inline void kexec_unlock(void)
 	atomic_set_release(&__kexec_lock, 0);
 }
 
+/*
+ * Different than kexec/kdump loading/unloading/crash or kexec jumping/shrinking
+ * which usually rarely happen, there will be many crash hotplug events notified
+ * during one short period, e.g one memory board is hot added and memory regions
+ * are online. So mutex lock  __crash_hotplug_lock is used to serialize the crash
+ * hotplug handling specificially.
+ * */
+extern struct mutex __crash_hotplug_lock;
+#define crash_hotplug_lock() mutex_lock(&__crash_hotplug_lock)
+#define crash_hotplug_unlock() mutex_unlock(&__crash_hotplug_lock)
+
 #ifdef CONFIG_KEXEC_FILE
 #include <linux/purgatory.h>
 void kimage_file_post_load_cleanup(struct kimage *image);
-- 
2.41.0