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Date:      Tue, 26 Jun 2018 08:56:34 +0000 (UTC)
From:      Andriy Gapon <avg@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-11@freebsd.org
Subject:   svn commit: r335658 - stable/11/sys/kern
Message-ID:  <201806260856.w5Q8uY5k086337@repo.freebsd.org>
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Author: avg
Date: Tue Jun 26 08:56:34 2018
New Revision: 335658
URL: https://svnweb.freebsd.org/changeset/base/335658

Log:
  MFC r333268: for bus suspend, detach and shutdown iterate children in reverse order

Modified:
  stable/11/sys/kern/subr_bus.c
Directory Properties:
  stable/11/   (props changed)

Modified: stable/11/sys/kern/subr_bus.c
==============================================================================
--- stable/11/sys/kern/subr_bus.c	Tue Jun 26 08:35:58 2018	(r335657)
+++ stable/11/sys/kern/subr_bus.c	Tue Jun 26 08:56:34 2018	(r335658)
@@ -3697,7 +3697,11 @@ bus_generic_detach(device_t dev)
 	if (dev->state != DS_ATTACHED)
 		return (EBUSY);
 
-	TAILQ_FOREACH(child, &dev->children, link) {
+	/*
+	 * Detach children in the reverse order.
+	 * See bus_generic_suspend for details.
+	 */
+	TAILQ_FOREACH_REVERSE(child, &dev->children, device_list, link) {
 		if ((error = device_detach(child)) != 0)
 			return (error);
 	}
@@ -3717,7 +3721,11 @@ bus_generic_shutdown(device_t dev)
 {
 	device_t child;
 
-	TAILQ_FOREACH(child, &dev->children, link) {
+	/*
+	 * Shut down children in the reverse order.
+	 * See bus_generic_suspend for details.
+	 */
+	TAILQ_FOREACH_REVERSE(child, &dev->children, device_list, link) {
 		device_shutdown(child);
 	}
 
@@ -3770,15 +3778,23 @@ int
 bus_generic_suspend(device_t dev)
 {
 	int		error;
-	device_t	child, child2;
+	device_t	child;
 
-	TAILQ_FOREACH(child, &dev->children, link) {
+	/*
+	 * Suspend children in the reverse order.
+	 * For most buses all children are equal, so the order does not matter.
+	 * Other buses, such as acpi, carefully order their child devices to
+	 * express implicit dependencies between them.  For such buses it is
+	 * safer to bring down devices in the reverse order.
+	 */
+	TAILQ_FOREACH_REVERSE(child, &dev->children, device_list, link) {
 		error = BUS_SUSPEND_CHILD(dev, child);
-		if (error) {
-			for (child2 = TAILQ_FIRST(&dev->children);
-			     child2 && child2 != child;
-			     child2 = TAILQ_NEXT(child2, link))
-				BUS_RESUME_CHILD(dev, child2);
+		if (error != 0) {
+			child = TAILQ_NEXT(child, link);
+			if (child != NULL) {
+				TAILQ_FOREACH_FROM(child, &dev->children, link)
+					BUS_RESUME_CHILD(dev, child);
+			}
 			return (error);
 		}
 	}

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