On Sun, 14 Dec 2003, Jeff Roberson wrote: > > On Sun, 14 Dec 2003, Jeff Roberson wrote: > > > On Sat, 13 Dec 2003, Don Lewis wrote: > > > > > On 13 Dec, Don Lewis wrote: > > > > On 12 Dec, Jeff Roberson wrote: > > > > > > > > > > > >> fsync: giving up on dirty: 0xc4e18000: tag devfs, type VCHR, usecount 44, > > > >> writecount 0, refcount 14, flags (VI_XLOCK|VV_OBJBUF), lock type devfs: EXCL > > > >> (count 1) by thread 0xc20ff500 > > > > > > > > Why are we trying to reuse a vnode with a usecount of 44 and a refcount > > > > of 14? What is thread 0xc20ff500 doing? > > > > > > Following up to myself ... > > > > > > It looks like we're trying to recycle this vnode because of the > > > following sysinstall code, in distExtractTarball(): > > > > > > if (is_base && RunningAsInit && !Fake) { > > > unmounted_dev = 1; > > > unmount("/dev", MNT_FORCE); > > > } else > > > unmounted_dev = 0; > > > > > > What happens if we forceably umount /dev while /dev/whatever holds a > > > mounted file system? It looks like this is handled by vgonechrl(). It > > > looks to me like vclean() is going to do some scary stuff to this vnode. > > > > > > > Excellent work! I think I may know what's wrong. If you look at rev > > 1.461 of vfs_subr.c I changed the semantics of cleaning a VCHR that was > > being unmounted. I now acquire the xlock around the operation. This may > > be the culprit. I'm too tired to debug this right now, but I can look at > > it in the am. > > > > Ok, I think I understand what happens.. The syncer runs, and at the same > time, we're doing the forced unmount. This causes the sync of the device > vnode to fail. This isn't really a problem. After this, while syncing > a ffs volume that is mounted on a VCHR from /dev, we bread() and get a > buffer for this device and then immediately block. The forced unmount > then proceeds, calling vclean() on the device, which goes into the VM via > DESTROYVOBJECT. The VM frees all of the pages associated with the object > etc. Then, the ffs_update() is allowed to run again with a pointer to a > buffer that has pointers to pages that have been freed. This is where > vfs_setdirty() comes in and finds a NULL object. > > The wired counts on the pages are 1, which is consistent with a page in > the bufcache. Also the object is NULL which is the only indication we > have that this is a free page. > > I think that if we want to allow unmounting of the underlying device for > VCHR, we need to not call vclean() from vgonechr(). We need to just lock, > VOP_RECLAIM, cache_purge(), and insmntque to NULL. > > I've looked through my changes here, and I don't see how I could have > introduced this bug. Were we vclean()ing before, and that seems to be the > main problem. There have been some changes to device aliasing that could > have impacted this. I'm trying to get the scoop from phk now. > > I'm going to change the way vgonechrl() works, but I'd really like to know > what changed that broke this.. > Please test the patch at: http://www.chesapeake.net/~jroberson/forcevchr.diff If this works I'll come up with a more compact arrangement for the code so that we can avoid all of this duplication. Cheers, Jeff > Cheers, > Jeff > > _______________________________________________ > freebsd-current_at_freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to "freebsd-current-unsubscribe_at_freebsd.org" >Received on Sun Dec 14 2003 - 09:41:17 UTC
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