[PATCH] direct-io: bug fix in dio handling write error

There is a bug in direct-io on propagating write error up to the higher I/O
layer.  When performing an async ODIRECT write to a block device, if a
device error occurred (like media error or disk is pulled), the error code
is only propagated from device driver to the DIO layer.  The error code
stops at finished_one_bio().  The aysnc write, however, is supposedly have
a corresponding AIO event with appropriate return code (in this case -EIO).
 Application which waits on the async write event, will hang forever since
such AIO event is lost forever (if such app did not use the timeout option
in io_getevents call.  Regardless, an AIO event is lost).

The discovery of above bug leads to another discovery of potential race
window with dio->result.  The fundamental problem is that dio->result is
overloaded with dual use: an indicator of fall back path for partial dio
write, and an error indicator used in the I/O completion path.  In the
event of device error, the setting of -EIO to dio->result clashes with
value used to track partial write that activates the fall back path.

It was also pointed out that it is impossible to use dio->result to track
partial write and at the same time to track error returned from device
driver.  Because direct_io_work can only determines whether it is a partial
write at the end of io submission and in mid stream of those io submission,
a return code could be coming back from the driver.  Thus messing up all
the subsequent logic.

Proposed fix is to separating out error code returned by the IO completion
path from partial IO submit tracking.  A new variable is added to dio
structure specifically to track io error returned in the completion path.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Acked-by: Zach Brown <zach.brown@oracle.com>
Acked-by: Suparna Bhattacharya <suparna@in.ibm.com>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

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