On Thu, 26 Jul 2012 18:36:07 -0400 "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > On Tue, Jul 24, 2012 at 05:28:02PM +0000, ZUIDAM, Hans wrote: > > Hi Bruce, > > > > Thanks for the clarification. > > > > (I'm repeating a lot of my original mail because of the Cc: list.) > > > > > J. Bruce Fields > > > I think that's right, though I'm curious how you're managing to hit > > > that case reliably every time. Or is this an intermittent failure? > > It's an intermittent failure, but with the procedure shown below it is > > fairly easy to reproduce. The actual problem we see in our product > > is because of the way external storage media are handled in user-land. > > > > 192.168.1.10# mount -t xfs /dev/sdcr/sda1 /mnt > > 192.168.1.10# exportfs 192.168.1.11:/mnt > > > > 192.168.1.11# mount 192.168.1.10:/mnt /mnt > > 192.168.1.11# umount /mnt > > > > 192.168.1.10# exportfs -u 192.168.1.11:/mnt > > 192.168.1.10# umount /mnt > > umount: can't umount /media/recdisk: Device or resource busy > > > > What I actually do is the mount/unmount on the client via ssh. That > > is a good way to trigger the problem. > > > > We see that during the un-export the NFS caches are not flushed > > properly which is why the final unmount fails. > > > > In net/sunrpc/cache.c the cache times (last_refresh, expiry_time, > > flush_time) are measured in seconds. If I understand the code somewhat > > then during an NFS un-export the is done by setting the flush_time to > > the current time. The cache_flush() is called. If in that same second > > last_refresh is set to the current time then the cached item is not > > flushed. This will subsequently cause un-mount to fail because there > > is still a reference to the mount point. > > > > > J. Bruce Fields > > > I ran across that recently while reviewing the code to fix a related > > > problem. I'm not sure what the best fix would be. > > > > > > Previously raised here: > > > > > > http://marc.info/?l=linux-nfs&m=133514319408283&w=2 > > > > The description in your mail does indeed looks the same as the problem > > that we see. > > > > >From reading the code in net/sunrpc/cache.c I get the impression that it is > > not really possible to reliably flush the caches for an un-exportfs such > > that after flushing they will not accept entries for the un-exported IP/mount > > point combination. > > Right. So, possible ideas, from that previous message: > > - As Neil suggests, modify exportfs to wait a second between > updating etab and flushing the cache. At that point any > entries still using the old information are at least a second > old. That may be adequate for your case, but if someone out > there is sensitive to the time required to unexport then that > will annoy them. It also leaves the small possibility of > races where an in-progress rpc may still be using an export at > the time you try to flush. > - Implement some new interface that you can use to flush the > cache and that doesn't return until in-progress rpc's > complete. Since it waits for rpc's it's not purely a "cache" > layer interface any more. So maybe something like > /proc/fs/nfsd/flush_exports. > - As a workaround requiring no code changes: unexport, then shut > down the server entirely and restart it. Clients will see > that as a reboot recovery event and recover automatically, but > applications may see delays while that happens. Kind of a big > hammer, but if unexporting while other exports are in use is > rare maybe it would be adequate for your case. That's a shame... I had originally intended "rpc.nfsd 0" to simple stop all threads and nothing else. Then you would be able to: rpc.nfsd 0 exportfs -f unmount rpc.nfsd 16 and have a nice fast race-free unmount. But commit e096bbc6488d3e49d476bf986d33752709361277 'fixed' that :-( I wonder if it can be resurrected ... maybe not worth the effort. The idea of a new interface to synchronise with all threads has potential and doesn't need to be at the nfsd level - it could be in sunrpc. Maybe it could be built into the current 'flush' interface. 1/ iterate through all no-sleeping threads setting a flag an increasing a counter. 2/ when a thread completes current request, if test_and_clear the flag, it atomic_dec_and_test the counter and then wakes up some wait_queue_head. 3/ 'flush'ing thread waits on the waut_queue_head for the counter to be 0. If you don't hate it I could possibly even provide some code. NeilBrown
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