On 10/15/2015 01:31 PM, Ilya Dryomov wrote: > On Thu, Oct 15, 2015 at 7:10 PM, Alex Elder <elder@xxxxxxxx> wrote: >> On 10/11/2015 01:03 PM, Ilya Dryomov wrote: >>> Currently we leak parent_spec and trigger a "parent reference >>> underflow" warning if rbd_dev_create() in rbd_dev_probe_parent() fails. >>> The problem is we take the !parent out_err branch and that only drops >>> refcounts; parent_spec that would've been freed had we called >>> rbd_dev_unparent() remains and triggers rbd_warn() in >>> rbd_dev_parent_put() - at that point we have parent_spec != NULL and >>> parent_ref == 0, so counter ends up being -1 after the decrement. >>> >>> Redo rbd_dev_probe_parent() to fix this. >> >> I'm just starting to look at this. >> >> My up-front problem is that I want to understand why >> it's OK to no longer grab references to the parent spec >> and the client before creating the new parent device. >> Is it simply because the rbd_dev that's the subject >> of this call is already holding a reference to both, >> so no need to get another? I think that's right, but >> you should say that in the explanation. It's a >> change that's independent of the other change (which >> you describe). > > I still grab references, I just moved that code after rbd_dev_create(). > Fundamentally rbd_dev_create() is just a memory allocation, so whether > we acquire references before or after doesn't matter, except that > acquiring them before complicates the error path. OK, I accept that. But looking at the patch alone it made me wonder whether grabbing the references was necessary before creating the parent device. I guess the main point is you should have mentioned it in your description. I'm going to take another look at the original patch, now that I have your explanation (below). Thanks. -Alex >> OK, onto the change you describe. >> >> You say that we get the underflow if rbd_dev_create() fails. >> >> At that point in the function, we have: >> parent == NULL >> rbd_dev->parent_spec != NULL >> parent_spec holds a new reference to that >> rbdc holds a new reference to the client >> >> rbd_dev_create() only returns NULL if the kzalloc() fails. >> And if so, we jump to out_err, take the !parent branch, >> and we drop the references we took in rbdc and parent_spec >> before returning. >> >> Where is the leak? (Actually, underflow means we're >> dropping more references than we took.) > > We enter rbd_dev_probe_parent(). If ->parent_spec != NULL (and > therefore has a refcount of 1), we bump refcounts and proceed to > allocating parent rbd_dev. If rbd_dev_create() fails, we drop > refcounts and exit rbd_dev_probe_parent() with ->parent_spec != NULL > and a refcount of 1 on that spec. We take err_out_probe in > rbd_dev_image_probe() and through rbd_dev_unprobe() end up in > rbd_dev_parent_put(). Now, ->parent_spec != NULL but ->parent_ref == 0 > because we never got to atomic_set(&rbd_dev->parent_ref, 1) in > rbd_dev_probe_parent(). Local counter ends up -1 after the decrement > and so instead of calling rbd_dev_unparent() which would have freed > ->parent_spec we issue an underflow warning and bail. ->parent_spec is > leaked. > > Thanks, > > Ilya > -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
