diff --git a/sys/compat/linuxkpi/common/include/linux/smp.h b/sys/compat/linuxkpi/common/include/linux/smp.h
index 290b91469b12..c6d011fceb5f 100644
--- a/sys/compat/linuxkpi/common/include/linux/smp.h
+++ b/sys/compat/linuxkpi/common/include/linux/smp.h
@@ -1,39 +1,51 @@
 /*-
  * Copyright (c) 2017 Mark Johnston <markj@FreeBSD.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in
  *    the documentation and/or other materials provided with the
  *    distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
  * $FreeBSD$
  */
 
 #ifndef _LINUXKPI_LINUX_SMP_H_
 #define	_LINUXKPI_LINUX_SMP_H_
 
+/*
+ * Important note about the use of the function provided below:
+ *
+ * The callback function passed to on_each_cpu() is called from a
+ * so-called critical section, and if you need a mutex you will have
+ * to rewrite the code to use native FreeBSD mtx spinlocks instead of
+ * the spinlocks provided by the LinuxKPI! Be very careful to not call
+ * any LinuxKPI functions inside the on_each_cpu()'s callback
+ * function, because they may sleep, unlike in native Linux.
+ *
+ * Enabling witness(4) when testing, can catch such issues.
+ */
 #define	on_each_cpu(cb, data, wait) ({				\
 	CTASSERT(wait);						\
 	linux_on_each_cpu(cb, data);				\
 })
 
 extern int	linux_on_each_cpu(void (*)(void *), void *);
 
 #endif /* _LINUXKPI_LINUX_SMP_H_ */