From da7985a02b212ef3c447e35ed8d168b9026dcb40 Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh@NetBSD.org>
Date: Tue, 16 Aug 2022 00:21:15 +0000
Subject: [PATCH] KERNEL_LOCK(9): Send an IPI to print holder's stack trace on
 spinout.

---
 sys/kern/kern_lock.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/sys/kern/kern_lock.c b/sys/kern/kern_lock.c
index c302a6cc517f..332f161d5f40 100644
--- a/sys/kern/kern_lock.c
+++ b/sys/kern/kern_lock.c
@@ -134,6 +134,22 @@ lockops_t _kernel_lock_ops = {
 	.lo_dump = _kernel_lock_dump,
 };
 
+#ifdef LOCKDEBUG
+
+#include <ddb/ddb.h>
+
+static void
+kernel_lock_trace_ipi(void *cookie)
+{
+
+	printf("%s[%d %s]: hogging kernel lock\n", cpu_name(curcpu()),
+	    curlwp->l_lid,
+	    curlwp->l_name ? curlwp->l_name : curproc->p_comm);
+	db_stacktrace();
+}
+
+#endif
+
 /*
  * Initialize the kernel lock.
  */
@@ -177,6 +193,7 @@ _kernel_lock(int nlocks)
 	LOCKSTAT_FLAG(lsflag);
 	struct lwp *owant;
 #ifdef LOCKDEBUG
+	static struct cpu_info *kernel_lock_holder;
 	u_int spins = 0;
 #endif
 	int s;
@@ -235,6 +252,11 @@ _kernel_lock(int nlocks)
 #ifdef LOCKDEBUG
 			if (SPINLOCK_SPINOUT(spins)) {
 				extern int start_init_exec;
+				ipi_msg_t msg = {
+					.func = kernel_lock_trace_ipi,
+				};
+				ipi_unicast(&msg, kernel_lock_holder);
+				ipi_wait(&msg);
 				if (start_init_exec)
 					_KERNEL_LOCK_ABORT("spinout");
 			}
@@ -278,6 +300,10 @@ _kernel_lock(int nlocks)
 #ifndef __HAVE_ATOMIC_AS_MEMBAR
 	membar_enter();
 #endif
+
+#ifdef LOCKDEBUG
+	kernel_lock_holder = curcpu();
+#endif
 }
 
 /*