svn commit: r255244 - head/sys/vm

[Konstantin Belousov]

Author: kib
Date: Thu Sep  5 12:54:40 2013
New Revision: 255244
URL: http://svnweb.freebsd.org/changeset/base/255244

Log:
  The vm_page_trysbusy() should not fail when shared busy counter or
  VPB_BIT_WAITERS flag were changed between reading of busy_lock and the
  cas.  The vm_page_sbusy(), which is the only user of
  vm_page_trysbusy() in the tree, panics on the failure, which in these
  cases is transient and do not mean that the current page state
  prevents sbusying.
  
  Retry the operation inside vm_page_trysbusy() if cas failed, only
  return a failure when VPB_BIT_SHARED is cleared.
  
  Reported and tested by:	pho
  Reviewed by:	attilio
  Sponsored by:	The FreeBSD Foundation

Modified:
  head/sys/vm/vm_page.c

Modified: head/sys/vm/vm_page.c
==============================================================================
--- head/sys/vm/vm_page.c	Thu Sep  5 12:35:23 2013	(r255243)
+++ head/sys/vm/vm_page.c	Thu Sep  5 12:54:40 2013	(r255244)
@@ -602,9 +602,13 @@ vm_page_trysbusy(vm_page_t m)
 {
 	u_int x;
 
-	x = m->busy_lock;
-	return ((x & VPB_BIT_SHARED) != 0 &&
-	    atomic_cmpset_acq_int(&m->busy_lock, x, x + VPB_ONE_SHARER));
+	for (;;) {
+		x = m->busy_lock;
+		if ((x & VPB_BIT_SHARED) == 0)
+			return (0);
+		if (atomic_cmpset_acq_int(&m->busy_lock, x, x + VPB_ONE_SHARER))
+			return (1);
+	}
 }
 
 /*