Re: [PATCH] ceph: fail the open_by_handle_at() if the dentry is being unlinked

[Sebastian Hasler <sebastian.hasler@xxxxxxxxxxxxxxxxxxxx>]

While reviewing the implementation of __fh_to_dentry (in the CephFS 
client), I noticed a possible race condition.

Linux has a syscall linkat(2) which allows, given an open file 
descriptor, to create a link for the file. So an inode that is unlinked 
can become linked.

Now the problem: The line ((inode->i_nlink == 0) && 
!__ceph_is_file_opened(ci)) performs two checks. If, in between those 
checks, the file goes from the unlinked and open state to the linked and 
closed state, then we return -ESTALE even though the inode is linked. I 
don't think this is the intended behavior. I guess this (going from 
unlinked and open to linked and closed) can happen when a concurrent 
process calls linkat() and then close().