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From: Attilio Rao <attilio@FreeBSD.org>
Date: Tue, 16 Dec 2008 23:16:10 +0000 (UTC)
To: cvs-src-old@freebsd.org
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Subject: cvs commit: src/sys/kern vfs_mount.c vfs_subr.c src/sys/sys mount.h
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attilio     2008-12-16 23:16:10 UTC

  FreeBSD src repository

  Modified files:
    sys/kern             vfs_mount.c vfs_subr.c 
    sys/sys              mount.h 
  Log:
  SVN rev 186197 on 2008-12-16 23:16:10Z by attilio
  
  1) Fix a deadlock in the VFS:
  - threadA runs vfs_rel(mp1)
  - threadB does unmount the mp1 fs, sets MNTK_UNMOUNT and drop MNT_ILOCK()
  - threadA runs vfs_busy(mp1) and, as long as, MNTK_UNMOUNT is set, sleeps
    waiting for threadB to complete the unmount
  - threadB, in vfs_mount_destroy(), finds mnt_lock > 0 and sleeps waiting
    for the refcount to expire.
  
  Fix the deadlock by adding a flag called MNTK_REFEXPIRE which signals the
  unmounter is waiting for mnt_ref to expire.
  The vfs_busy contenders got awake, fails, and if they retry the
  MNTK_REFEXPIRE won't allow them to sleep again.
  
  2) Simplify significantly the code of vfs_mount_destroy() trimming
     unnecessary codes:
     - as long as any reference exited, it is no-more possible to have
       write-op (primarty and secondary) in progress.
     - it is no needed to drop and reacquire the mount lock.
     - filling the structures with dummy values is unuseful as long as
       it is going to be freed.
  
  Tested by:      pho, Andrea Barberio <insomniac at slackware dot it>
  Discussed with: kib
  
  Revision  Changes    Path
  1.298     +14 -31    src/sys/kern/vfs_mount.c
  1.750     +1 -1      src/sys/kern/vfs_subr.c
  1.240     +1 -0      src/sys/sys/mount.h