rbd: do a safe list traversal in rbd_img_request_submit()

It's possible that the reference to the object request dropped
inside the loop in rbd_img_request_submit() will be the last
one, in which case the content of the object pointer can't be
trusted.

Use a safe form of the object request list traversal to avoid
problems.

This resolves:
    http://tracker.ceph.com/issues/4705

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: record aggregate image request result

If any image object request produces a non-zero result, preserve
that as the result of the overall image request.  If multiple
objects have non-zero results, save only the first one.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: record aggregate image transfer count

Compute the total number of bytes transferred for an image
request--the sum across each of the request's object requests.
To avoid contention do it only when all object requests are
complete, in rbd_img_request_complete().

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: record image-relative offset in object request

For an image object request we will need to know what offset within
the rbd image the request covers.  Record that when the object
request gets created.

Update the I/O error warnings so they use this so what's reported
is more informative.

Rename a local variable to fit the convention used everywhere else.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: define image request flags

There are several Boolean values we'll be maintaining for image
requests.  Switch from the single write_request field to a
general-purpose flags field, and use one if its bits to represent
the direction of I/O for the image request.  Define helper functions
for setting and testing that flag.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: define image request originator flag

Define a flag indicating whether an image request originated from
the Linux block layer (from blk_fetch_request()) or whether it was
initiated in order to satisfy an object request for a child image
of a layered rbd device.  For image requests initiated by objects of
child images we'll save a pointer to the object request rather than
the Linux block request.

For now, only block requests are used.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: define image request layered flag

Define a flag indicating whether an image request is for a layered
image (one with a parent image to which requests will be redirected
if the target object of a request does not exist).  The code that
checks this flag will be added shortly.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: encapsulate image object end request handling

Encapsulate the code that completes processing of an object request
that's part of an image request.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: temp layering support

Pretend we support layering...

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: probe the parent of an image if present

Call the probe function for the parent device.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: define rbd object request flags field

We're going to need some more Boolean values for object requests,
so create a flags bit field and use it to record whether the request
is done.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: add object request flag for image data objects

Add a flag to distinguish between object requests being done on
standalone objects and requests being sent for objects representing
rbd image data (i.e., object requests that are the result of image
request).

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: implement layered reads

Still working on this.

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: revalidate_disk upon rbd resize

If rbd disk is open and rbd resize is done, new size is not
visible by filesystem.  Like is done in virtio-blk and dm driver,
revalidate_disk() permits to update the bd_inode size.

Signed-off-by: Laurent Barbe <laurent@ksperis.com>
Reviewed-by: Alex Elder <elder@inktank.com>