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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Sun, 22 Jan 2017 19:41:42 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r312647 - head/sys/kern
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Author: kib
Date: Sun Jan 22 19:41:42 2017
New Revision: 312647
URL: https://svnweb.freebsd.org/changeset/base/312647

Log:
  Add comments explaining unobvious td_critnest adjustments in
  critical_exit().
  
  Based on the discussion with:	jhb
  Reviewed by:	imp
  Sponsored by:	The FreeBSD Foundation
  Differential revision:	D9276
  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 Jan 22 19:38:45 2017	(r312646)
+++ head/sys/kern/kern_switch.c	Sun Jan 22 19:41:42 2017	(r312647)
@@ -206,7 +206,22 @@ critical_exit(void)
 
 	if (td->td_critnest == 1) {
 		td->td_critnest = 0;
+
+		/*
+		 * Interrupt handlers execute critical_exit() on
+		 * leave, and td_owepreempt may be left set by an
+		 * interrupt handler only when td_critnest > 0.  If we
+		 * are decrementing td_critnest from 1 to 0, read
+		 * td_owepreempt after decrementing, to not miss the
+		 * preempt.  Disallow compiler to reorder operations.
+		 */
+		__compiler_membar();
 		if (td->td_owepreempt && !kdb_active) {
+			/*
+			 * Microoptimization: we committed to switch,
+			 * disable preemption in interrupt handlers
+			 * while spinning for the thread lock.
+			 */
 			td->td_critnest = 1;
 			thread_lock(td);
 			td->td_critnest--;