loop: it looks like REQ_OP_FLUSH could return before IO completion.

[Eric Wheeler <linux-block@lists.ewheeler.net>]

Hello all,

In loop.c do_req_filebacked() for REQ_OP_FLUSH, lo_req_flush() is called: 
it does not appear that lo_req_flush() does anything to make sure 
ki_complete has been called for pending work, it just calls vfs_fsync().

Is this a consistency problem?

For example, if the loop driver has queued async kiocb's to the filesystem 
via .read_iter/.write_iter, is it the filesystem's responsibility to 
complete that work before returning from vfs_sync() or is it possible that 
the vfs_sync() completes before ->ki_complete() is called?

Relatedly, does vfs_fsync() do anything for direct IO?  Ultimately 
f_op->fsync() is called so maybe the FS is told to commit its structures 
like sparse allocations that may not be on disk yet.


--
Eric Wheeler