[PATCH v3] Crash: add lock to serialize crash hotplug handling

[Baoquan He]

Eric reported that handling corresponding crash hotplug event can be
failed easily when many memory 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 at redhat.com>
---
v2->v3:
 - crash_check_update_elfcorehdr() need take __crash_hotplug_lock
   too because there's tiny racing window when kexec_load interface
   is taken. Eric pointed out this.
v1->v2:
 - Move mutex lock definition into CONFIG_CRASH_HOTPLUG ifdeffery
   scope in kernel/crash_core.c because the lock is only needed and
   used in that scope. Suggested by Eric.
 kernel/crash_core.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 03a7932cde0a..2f675ef045d4 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -739,6 +739,17 @@ subsys_initcall(crash_notes_memory_init);
 #undef pr_fmt
 #define pr_fmt(fmt) "crash hp: " fmt
 
+/*
+ * Different than kexec/kdump loading/unloading/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 specifically.
+ */
+DEFINE_MUTEX(__crash_hotplug_lock);
+#define crash_hotplug_lock() mutex_lock(&__crash_hotplug_lock)
+#define crash_hotplug_unlock() mutex_unlock(&__crash_hotplug_lock)
+
 /*
  * This routine utilized when the crash_hotplug sysfs node is read.
  * It reflects the kernel's ability/permission to update the crash
@@ -748,9 +759,11 @@ int crash_check_update_elfcorehdr(void)
 {
 	int rc = 0;
 
+	crash_hotplug_lock();
 	/* Obtain lock while reading crash information */
 	if (!kexec_trylock()) {
 		pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
+		crash_hotplug_unlock();
 		return 0;
 	}
 	if (kexec_crash_image) {
@@ -761,6 +774,7 @@ int crash_check_update_elfcorehdr(void)
 	}
 	/* Release lock now that update complete */
 	kexec_unlock();
+	crash_hotplug_unlock();
 
 	return rc;
 }
@@ -783,9 +797,11 @@ 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 +868,7 @@ 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)
-- 
2.41.0