Public bug reported: I'm filing this bug a little prematurely because Abhi and I didn't get a chance to fully discuss it. However, looking at the code and the behavior I'm seeing due to another bug (1884587), I feel rather confident.
Especially in a situation where glance is running on multiple control plane nodes (i.e. any real-world situation), I believe there is a race condition whereby two closely-timed requests to copy an image to a store will result in two copy operations in glance proceeding in parallel. I believe this to be the case due to a common "test-and-set that isn't atomic" error. In the API layer, glance checks that an import copy-to-store operation isn't already in progress here: https://github.com/openstack/glance/blob/e6db0b10a703037f754007bef6f56451086850cd/glance/api/v2/images.py#L167 And if that passes, it proceeds to setup the task as a thread here: https://github.com/openstack/glance/blob/e6db0b10a703037f754007bef6f56451086850cd/glance/api/v2/images.py#L197 which may start running immediately or sometime in the future. Once running, that code updates a property on the image to indicate that the task is running here: https://github.com/openstack/glance/blob/e6db0b10a703037f754007bef6f56451086850cd/glance/async_/flows/api_image_import.py#L479-L484 Between those two events, if another API user makes the same request, glance will not realize that a thread is already running to complete the initial task and will start another. In a situation where a user spawns a thousand new instances to a thousand compute nodes in a single operation where the image needs copying first, it's highly plausible to have _many_ duplicate glance operations going, impacting write performance on the rbd cluster at the very least. As evidence that this can happen, we see an abnormally extended race window because of the aforementioned bug (1884587) where we fail to update the property that indicates the task is running. In a test we see a large number of them get started, followed by a cascade of failures when they fail to update that image property, implying that many such threads are running. If this situation is allowed to happen when the property does *not* fail to update, I believe we would end up with glance copying the image to the destination in multiple threads simultaneously. That is much harder to simulate in practice in a development environment, but the other bug makes it happen every time since we never update the image property to prevent it and thus the window is long. Abhi also brought up the case where if this race occurs on the same node, the second attempt *may* actually start copying the partial image in the staging directory to the destination, finish early, and then mark the image as "copied to $store" such that nova will attempt to use the partial image immediately, resulting in a corrupted disk and various levels of failure after that. Note that it's not clear if that's really possible or not, but I'm putting it here so the glance gurus can validate. The use of the os_glance_importing_to_stores property to "lock" a copy to a particular store is good, except that updating that list atomically means that the above mentioned race will not have anything to check after the update to see if it was the race loser. I don't see any checks in the persistence layer to ensure that an UPDATE to the row with this property doesn't already have a given store in it, or do any kind of merge. This also leads me to worry that two parallel requests to copy an image to two different stores may result in clobbering the list of stores-in-progress and potentially also the final list of stores at rest. This is just conjecture at this point, I just haven't seen anywhere that situation is accounted for. ** Affects: glance Importance: Undecided Status: New -- You received this bug notification because you are a member of Yahoo! Engineering Team, which is subscribed to Glance. https://bugs.launchpad.net/bugs/1884596 Title: image import copy-to-store will start multiple importing threads due to race condition Status in Glance: New Bug description: I'm filing this bug a little prematurely because Abhi and I didn't get a chance to fully discuss it. However, looking at the code and the behavior I'm seeing due to another bug (1884587), I feel rather confident. Especially in a situation where glance is running on multiple control plane nodes (i.e. any real-world situation), I believe there is a race condition whereby two closely-timed requests to copy an image to a store will result in two copy operations in glance proceeding in parallel. I believe this to be the case due to a common "test-and-set that isn't atomic" error. In the API layer, glance checks that an import copy-to-store operation isn't already in progress here: https://github.com/openstack/glance/blob/e6db0b10a703037f754007bef6f56451086850cd/glance/api/v2/images.py#L167 And if that passes, it proceeds to setup the task as a thread here: https://github.com/openstack/glance/blob/e6db0b10a703037f754007bef6f56451086850cd/glance/api/v2/images.py#L197 which may start running immediately or sometime in the future. Once running, that code updates a property on the image to indicate that the task is running here: https://github.com/openstack/glance/blob/e6db0b10a703037f754007bef6f56451086850cd/glance/async_/flows/api_image_import.py#L479-L484 Between those two events, if another API user makes the same request, glance will not realize that a thread is already running to complete the initial task and will start another. In a situation where a user spawns a thousand new instances to a thousand compute nodes in a single operation where the image needs copying first, it's highly plausible to have _many_ duplicate glance operations going, impacting write performance on the rbd cluster at the very least. As evidence that this can happen, we see an abnormally extended race window because of the aforementioned bug (1884587) where we fail to update the property that indicates the task is running. In a test we see a large number of them get started, followed by a cascade of failures when they fail to update that image property, implying that many such threads are running. If this situation is allowed to happen when the property does *not* fail to update, I believe we would end up with glance copying the image to the destination in multiple threads simultaneously. That is much harder to simulate in practice in a development environment, but the other bug makes it happen every time since we never update the image property to prevent it and thus the window is long. Abhi also brought up the case where if this race occurs on the same node, the second attempt *may* actually start copying the partial image in the staging directory to the destination, finish early, and then mark the image as "copied to $store" such that nova will attempt to use the partial image immediately, resulting in a corrupted disk and various levels of failure after that. Note that it's not clear if that's really possible or not, but I'm putting it here so the glance gurus can validate. The use of the os_glance_importing_to_stores property to "lock" a copy to a particular store is good, except that updating that list atomically means that the above mentioned race will not have anything to check after the update to see if it was the race loser. I don't see any checks in the persistence layer to ensure that an UPDATE to the row with this property doesn't already have a given store in it, or do any kind of merge. This also leads me to worry that two parallel requests to copy an image to two different stores may result in clobbering the list of stores-in-progress and potentially also the final list of stores at rest. This is just conjecture at this point, I just haven't seen anywhere that situation is accounted for. To manage notifications about this bug go to: https://bugs.launchpad.net/glance/+bug/1884596/+subscriptions -- Mailing list: https://launchpad.net/~yahoo-eng-team Post to : [email protected] Unsubscribe : https://launchpad.net/~yahoo-eng-team More help : https://help.launchpad.net/ListHelp
