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From: Alan Cox <alc@FreeBSD.org>
Date: Sat, 15 Jan 2005 21:12:47 +0000 (UTC)
To: src-committers@FreeBSD.org, cvs-src@FreeBSD.org,
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X-FreeBSD-CVS-Branch: HEAD
Subject: cvs commit: src/sys/vm vm_object.c
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alc         2005-01-15 21:12:47 UTC

  FreeBSD src repository

  Modified files:
    sys/vm               vm_object.c 
  Log:
  Consider three objects, O, BO, and BBO, where BO is O's backing object
  and BBO is BO's backing object.  Now, suppose that O and BO are being
  collapsed.  Furthermore, suppose that BO has been marked dead
  (OBJ_DEAD) by vm_object_backing_scan() and that either
  vm_object_backing_scan() has been forced to sleep due to encountering
  a busy page or vm_object_collapse() has been forced to sleep due to
  memory allocation in the swap pager.  If vm_object_deallocate() is
  then called on BBO and BO is BBO's only shadow object,
  vm_object_deallocate() will collapse BO and BBO.  In doing so, it adds
  a necessary temporary reference to BO.  If this collapse also sleeps
  and the prior collapse resumes first, the temporary reference will
  cause vm_object_collapse to panic with the message "backing_object %p
  was somehow re-referenced during collapse!"
  
  Resolve this race by changing vm_object_deallocate() such that it
  doesn't collapse BO and BBO if BO is marked dead.  Once O and BO are
  collapsed, vm_object_collapse() will attempt to collapse O and BBO.
  So, vm_object_deallocate() on BBO need do nothing.
  
  Reported by: Peter Holm on 20050107
  URL: http://www.holm.cc/stress/log/cons102.html
  
  In collaboration with: tegge@
  Candidate for RELENG_4 and RELENG_5
  MFC after: 2 weeks
  
  Revision  Changes    Path
  1.343     +8 -1      src/sys/vm/vm_object.c