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possible fix for PR 39307 (mfs sometimes crashing on unmount)


I have been puzzling over the symptoms of PR # 39307 for a while, mfs
sometimes causes a panic in VFS_START.  Looking at it, mfs is a
strange beast.  The start routine (mfs_start) does not exit until the
mount is going away and then mfs_start() actually calls dounmount()
directly to unmount itself - once this is done, the race is on, can
mfs_start() exit and VFS_START() do it's final bit before the memory
that was the struct mount * is reused.  Hence the crash.

I was able to easily reproduce this panic every time using qemu.  I
have found what I think is a fix but I am not sure it is *correct*.
The panic is due to the mnt_refcnt being wrong, there are too few
references to the mount structure, when mfs_start() calls dounmount()
there are two vfs_destroy() calls near the end of dounmount() - the
comments say one is to clear the caller's reference and the other is
for mount().  Well, mfs_start() does not increment the mnt_refcnt
before calling dounmount() which causes the ensuing chaos.  The
following fixes the symptom but I don't know if it is the "right" way
to fix the bug:

Index: mfs_vfsops.c
RCS file: /cvsroot/src/sys/ufs/mfs/mfs_vfsops.c,v
retrieving revision 1.98
diff -u -r1.98 mfs_vfsops.c
--- mfs_vfsops.c        28 Jun 2008 01:34:05 -0000      1.98
+++ mfs_vfsops.c        25 Sep 2008 12:23:29 -0000
@@ -403,7 +403,7 @@
-       vfs_unbusy(mp, false, NULL);
+       vfs_unbusy(mp, true, NULL);
        base = mfsp->mfs_baseoff;

Brett Lymn
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