Record the file, line, and pid of the last successful shared lock holder. This

is useful as a last effort in debugging file system deadlocks.  This is enabled
via 'options DEBUG_LOCKS'

sys/kern/kern_lock.c

@@ -259,6 +259,12 @@ debuglockmgr(lkp, flags, interlkp, td, name, file, line)
 			if (error)
 				break;
 			sharelock(lkp, 1);
+#if defined(DEBUG_LOCKS)
+			lkp->lk_slockholder = pid;
+			lkp->lk_sfilename = file;
+			lkp->lk_slineno = line;
+			lkp->lk_slockername = name;
+#endif
 			break;
 		}
 		/*

sys/sys/lockmgr.h

@@ -62,6 +62,11 @@ struct lock {
 	const char *lk_filename;
 	const char *lk_lockername;
 	int     lk_lineno;
+
+	pid_t	lk_slockholder;
+	const char *lk_sfilename;
+	const char *lk_slockername;
+	int     lk_slineno;
 #endif
 };
 /*