Intermittent permission denied using kernel client with mds path cap

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Hi all, Hi Zheng,

We're seeing a strange issue with the kernel cephfs clients, combined
with a path restricted mds cap. It seems that files/dirs are
intermittently not created due to permission denied.

For example, when I untar a kernel into cephfs, we see ~1/1000 files
failed to open/mkdir.
Client caps are in the PS [1].
We've tried kernels 3.10.0-493.el7, 4.8.6, and 4.9-rc4 -- all have the
same intermittent behaviour. We could *not* reproduce the issue with
ceph-fuse 10.2.3.

The cluster is running 10.2.3.

Now, if we remove the path restricted cap -- i.e. use mds 'allow rw'
-- then we have no more errors.

So it seems there is a race in the path restriction cap code.
We grabbed an mds log, and noticed that it seems that a file is opened
twice, then the second open fails with 'already xlocked'. A full log
for one such file is here: http://pastebin.com/raw/YyULfjND

Is anyone successfully using path caps with kernel clients? Maybe this
is a new bug?

Cheers, Dan


[1]
[client.k8s]
   key: xxx==
   caps: [mds] allow r, allow rw path=/k8s
   caps: [mon] allow r
   caps: [osd] allow rw pool=cephfs_data_k8s
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