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rbd: fix rbd_dev_parent_get() when parent_overlap == 0 

The comment for rbd_dev_parent_get() said

    * We must get the reference before checking for the overlap to
    * coordinate properly with zeroing the parent overlap in
    * rbd_dev_v2_parent_info() when an image gets flattened.  We
    * drop it again if there is no overlap.

but the "drop it again if there is no overlap" part was missing from
the implementation.  This lead to absurd parent_ref values for images
with parent_overlap == 0, as parent_ref was incremented for each
img_request and virtually never decremented.

Fix this by leveraging the fact that refresh path calls
rbd_dev_v2_parent_info() under header_rwsem and use it for read in
rbd_dev_parent_get(), instead of messing around with atomics.  Get rid
of barriers in rbd_dev_v2_parent_info() while at it - I don't see what
they'd pair with now and I suspect we are in a pretty miserable
situation as far as proper locking goes regardless.

Cc: stable@vger.kernel.org # 3.11+
Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Reviewed-by: Josh Durgin <jdurgin@redhat.com>
Reviewed-by: Alex Elder <elder@linaro.org>
This commit is contained in:
Ilya Dryomov 2015-01-19 18:13:43 +03:00
parent 26bc420b59
commit ae43e9d05e
1 changed files with 6 additions and 14 deletions
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20
drivers/block/rbd.c
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@ -2098,32 +2098,26 @@ static void rbd_dev_parent_put(struct rbd_device *rbd_dev)
* If an image has a non-zero parent overlap, get a reference to its
* parent.
*
* We must get the reference before checking for the overlap to
* coordinate properly with zeroing the parent overlap in
* rbd_dev_v2_parent_info() when an image gets flattened. We
* drop it again if there is no overlap.
*
* Returns true if the rbd device has a parent with a non-zero
* overlap and a reference for it was successfully taken, or
* false otherwise.
*/
static bool rbd_dev_parent_get(struct rbd_device *rbd_dev)
{
int counter;
int counter = 0;
if (!rbd_dev->parent_spec)
return false;
counter = atomic_inc_return_safe(&rbd_dev->parent_ref);
if (counter > 0 && rbd_dev->parent_overlap)
return true;
/* Image was flattened, but parent is not yet torn down */
down_read(&rbd_dev->header_rwsem);
if (rbd_dev->parent_overlap)
counter = atomic_inc_return_safe(&rbd_dev->parent_ref);
up_read(&rbd_dev->header_rwsem);
if (counter < 0)
rbd_warn(rbd_dev, "parent reference overflow");
return false;
return counter > 0;
}
/*
@ -4239,7 +4233,6 @@ static int rbd_dev_v2_parent_info(struct rbd_device *rbd_dev)
*/
if (rbd_dev->parent_overlap) {
rbd_dev->parent_overlap = 0;
smp_mb();
rbd_dev_parent_put(rbd_dev);
pr_info("%s: clone image has been flattened\n",
rbd_dev->disk->disk_name);
@ -4285,7 +4278,6 @@ static int rbd_dev_v2_parent_info(struct rbd_device *rbd_dev)
* treat it specially.
*/
rbd_dev->parent_overlap = overlap;
smp_mb();
if (!overlap) {
/* A null parent_spec indicates it's the initial probe */