From 0f2d5be792b0466b06797f637cfbb0f64dbb408c Mon Sep 17 00:00:00 2001 From: Alex Elder Date: Sat, 26 Apr 2014 14:21:44 +0400 Subject: [PATCH] rbd: use reference counts for image requests Each image request contains a reference count, but to date it has not actually been used. (I think this was just an oversight.) A recent report involving rbd failing an assertion shed light on why and where we need to use these reference counts. Every OSD request associated with an object request uses rbd_osd_req_callback() as its callback function. That function will call a helper function (dependent on the type of OSD request) that will set the object request's "done" flag if the object request if appropriate. If that "done" flag is set, the object request is passed to rbd_obj_request_complete(). In rbd_obj_request_complete(), requests are processed in sequential order. So if an object request completes before one of its predecessors in the image request, the completion is deferred. Otherwise, if it's a completing object's "turn" to be completed, it is passed to rbd_img_obj_end_request(), which records the result of the operation, accumulates transferred bytes, and so on. Next, the successor to this request is checked and if it is marked "done", (deferred) completion processing is performed on that request, and so on. If the last object request in an image request is completed, rbd_img_request_complete() is called, which (typically) destroys the image request. There is a race here, however. The instant an object request is marked "done" it can be provided (by a thread handling completion of one of its predecessor operations) to rbd_img_obj_end_request(), which (for the last request) can then lead to the image request getting torn down. And this can happen *before* that object has itself entered rbd_img_obj_end_request(). As a result, once it *does* enter that function, the image request (and even the object request itself) may have been freed and become invalid. All that's necessary to avoid this is to properly count references to the image requests. We tear down an image request's object requests all at once--only when the entire image request has completed. So there's no need for an image request to count references for its object requests. However, we don't want an image request to go away until the last of its object requests has passed through rbd_img_obj_callback(). In other words, we don't want rbd_img_request_complete() to necessarily result in the image request being destroyed, because it may get called before we've finished processing on all of its object requests. So the fix is to add a reference to an image request for each of its object requests. The reference can be viewed as representing an object request that has not yet finished its call to rbd_img_obj_callback(). That is emphasized by getting the reference right after assigning that as the image object's callback function. The corresponding release of that reference is done at the end of rbd_img_obj_callback(), which every image object request passes through exactly once. Cc: stable@vger.kernel.org Signed-off-by: Alex Elder Reviewed-by: Ilya Dryomov --- drivers/block/rbd.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c index 94747afc2e78..34a981ba1b9e 100644 --- a/drivers/block/rbd.c +++ b/drivers/block/rbd.c @@ -1382,6 +1382,13 @@ static void rbd_obj_request_put(struct rbd_obj_request *obj_request) kref_put(&obj_request->kref, rbd_obj_request_destroy); } +static void rbd_img_request_get(struct rbd_img_request *img_request) +{ + dout("%s: img %p (was %d)\n", __func__, img_request, + atomic_read(&img_request->kref.refcount)); + kref_get(&img_request->kref); +} + static bool img_request_child_test(struct rbd_img_request *img_request); static void rbd_parent_request_destroy(struct kref *kref); static void rbd_img_request_destroy(struct kref *kref); @@ -2142,6 +2149,7 @@ static void rbd_img_obj_callback(struct rbd_obj_request *obj_request) img_request->next_completion = which; out: spin_unlock_irq(&img_request->completion_lock); + rbd_img_request_put(img_request); if (!more) rbd_img_request_complete(img_request); @@ -2242,6 +2250,7 @@ static int rbd_img_request_fill(struct rbd_img_request *img_request, goto out_unwind; obj_request->osd_req = osd_req; obj_request->callback = rbd_img_obj_callback; + rbd_img_request_get(img_request); if (write_request) { osd_req_op_alloc_hint_init(osd_req, which, -- GitLab