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From: Attilio Rao <attilio@FreeBSD.org>
Date: Fri, 22 Mar 2013 16:44:16 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-user@freebsd.org
Subject: svn commit: r248618 - user/attilio/vmobj-readlock/sys/vm
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Author: attilio
Date: Fri Mar 22 16:44:15 2013
New Revision: 248618
URL: http://svnweb.freebsd.org/changeset/base/248618

Log:
  Fix a problem when the page identity was changing after sleeping
  in vm_page_sleep* primitives.
  The lock can change as the page's object lock is not held, relocking
  then the wrong object in the end.
  
  Sponsored by:	EMC / Isilon storage division
  Reported and tested by:	pho

Modified:
  user/attilio/vmobj-readlock/sys/vm/vm_page.c

Modified: user/attilio/vmobj-readlock/sys/vm/vm_page.c
==============================================================================
--- user/attilio/vmobj-readlock/sys/vm/vm_page.c	Fri Mar 22 14:10:15 2013	(r248617)
+++ user/attilio/vmobj-readlock/sys/vm/vm_page.c	Fri Mar 22 16:44:15 2013	(r248618)
@@ -757,12 +757,21 @@ vm_page_readahead_finish(vm_page_t m)
 int
 vm_page_sleep_if_busy(vm_page_t m, int also_m_busy, const char *msg)
 {
+	vm_object_t obj;
 
 	VM_OBJECT_ASSERT_WLOCKED(m->object);
 	if ((m->oflags & VPO_BUSY) || (also_m_busy && m->busy)) {
-		VM_OBJECT_WUNLOCK(m->object);
+		/*
+		 * The page-specific object must be cached because page
+		 * identity can change during the sleep, causing the
+		 * re-lock of a different object.
+		 * It is assumed that a reference to the object is already
+		 * held by the callers.
+		 */
+		obj = m->object;
+		VM_OBJECT_WUNLOCK(obj);
 		vm_page_sleep(m, msg);
-		VM_OBJECT_WLOCK(m->object);
+		VM_OBJECT_WLOCK(obj);
 		return (TRUE);
 	}
 	return (FALSE);