[RFC PATCH] spinlock_debug: Save stacktrace at lock acquisition

[Guru Das Srinagesh]

In case of spinlock recursion bugs, knowing which entity acquired the
lock initially is key to debugging. Therefore, save the stack frames
when the lock is acquired and print them when spinlock recursion is
detected.

Signed-off-by: Guru Das Srinagesh <gurus@xxxxxxxxxxxxxx>
---
 include/linux/spinlock_types.h  | 3 +++
 kernel/locking/spinlock_debug.c | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/spinlock_types.h b/include/linux/spinlock_types.h
index b981caa..8a68d55 100644
--- a/include/linux/spinlock_types.h
+++ b/include/linux/spinlock_types.h
@@ -22,6 +22,9 @@ typedef struct raw_spinlock {
 #ifdef CONFIG_DEBUG_SPINLOCK
 	unsigned int magic, owner_cpu;
 	void *owner;
+#define MAX_STACK_LEN 16
+	int stack_len;
+	unsigned long stack_trace[MAX_STACK_LEN];
 #endif
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index b9d9308..d34cccc 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -12,6 +12,7 @@
 #include <linux/debug_locks.h>
 #include <linux/delay.h>
 #include <linux/export.h>
+#include <linux/stacktrace.h>
 
 void __raw_spin_lock_init(raw_spinlock_t *lock, const char *name,
 			  struct lock_class_key *key, short inner)
@@ -27,6 +28,7 @@ void __raw_spin_lock_init(raw_spinlock_t *lock, const char *name,
 	lock->magic = SPINLOCK_MAGIC;
 	lock->owner = SPINLOCK_OWNER_INIT;
 	lock->owner_cpu = -1;
+	lock->stack_len = 0;
 }
 
 EXPORT_SYMBOL(__raw_spin_lock_init);
@@ -65,6 +67,10 @@ static void spin_dump(raw_spinlock_t *lock, const char *msg)
 		owner ? task_pid_nr(owner) : -1,
 		READ_ONCE(lock->owner_cpu));
 	dump_stack();
+	if (!strcmp(msg, "recursion")) {
+		printk(KERN_EMERG "Stack at lock acquisition: \n");
+		stack_trace_print(lock->stack_trace, lock->stack_len, 0);
+	}
 }
 
 static void spin_bug(raw_spinlock_t *lock, const char *msg)
@@ -90,6 +96,8 @@ static inline void debug_spin_lock_after(raw_spinlock_t *lock)
 {
 	WRITE_ONCE(lock->owner_cpu, raw_smp_processor_id());
 	WRITE_ONCE(lock->owner, current);
+	lock->stack_len = stack_trace_save(lock->stack_trace,
+					   ARRAY_SIZE(lock->stack_trace), 0);
 }
 
 static inline void debug_spin_unlock(raw_spinlock_t *lock)
@@ -101,6 +109,7 @@ static inline void debug_spin_unlock(raw_spinlock_t *lock)
 							lock, "wrong CPU");
 	WRITE_ONCE(lock->owner, SPINLOCK_OWNER_INIT);
 	WRITE_ONCE(lock->owner_cpu, -1);
+	WRITE_ONCE(lock->stack_len, 0);
 }
 
 /*
-- 
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