svn commit: r244046 - head/sys/kern

[Attilio Rao]

Author: attilio
Date: Sun Dec  9 04:54:22 2012
New Revision: 244046
URL: http://svnweb.freebsd.org/changeset/base/244046

Log:
  Add a comment on why inlining critical_enter() may not be a good idea
  for the general case.
  
  Reviewed by:	bde
  MFC after:	1 week

Modified:
  head/sys/kern/kern_switch.c

Modified: head/sys/kern/kern_switch.c
==============================================================================
--- head/sys/kern/kern_switch.c	Sun Dec  9 04:15:51 2012	(r244045)
+++ head/sys/kern/kern_switch.c	Sun Dec  9 04:54:22 2012	(r244046)
@@ -176,6 +176,12 @@ retry:
 /*
  * Kernel thread preemption implementation.  Critical sections mark
  * regions of code in which preemptions are not allowed.
+ *
+ * It might seem a good idea to inline critical_enter() but, in order
+ * to prevent instructions reordering by the compiler, a __compiler_membar()
+ * would have to be used here (the same as sched_pin()).  The performance
+ * penalty imposed by the membar could, then, produce slower code than
+ * the function call itself, for most cases.
  */
 void
 critical_enter(void)