[os-brick][nova][cinder][glance] Question about lock_path

Ben Nemec openstack at nemebean.com
Tue Feb 1 17:07:07 UTC 2022

General question: Are file locks even sufficient for this? If we're 
talking about multiple services coordinating access to a resource, do we 
know for certain that all of the processes accessing the resource are on 
the same system? If not, file locks can't solve this anyway.

If so, see my thoughts below.

On 2/1/22 09:57, Balazs Gibizer wrote:
> On Tue, Feb 1 2022 at 11:50:28 AM +0100, Radosław Piliszek 
> <radoslaw.piliszek at gmail.com> wrote:
>> Hello Fellow OpenStackers!
>> I have noticed os-brick started using the file locks (in lock_path) to
>> avoid race conditions.
>> I have a question regarding that lock_path - it seems all the cases I
>> have found use separate lock_paths per service, yet the problematic
>> cases listed in the bug report (and the commit message of [1]) suggest
>> that, in case of co-hosted nova-compute, glance-api, cinder-volume and
>> cinder-backup services (or any combination thereof), the lock_path
>> should be shared.
>> Should the deployment projects adapt to actually fix the race 
>> condition issue?
> An alternative to that would be that os-brick has specific lock path 
> config option regardless of which service is importing the lib. Then the 
> oslo_concurrency.lockutils.lock() can be called with that os-brick 
> specific path from os-brick.

The lock_path config opt is owned by oslo.concurrency so I don't think 
this is possible today. The simplest and least error-prone solution is 
to configure all of the users of os-brick to use the same lock path.

Even if we provided a way to override the lock path for just os-brick, 
you'd still need to add a config opt to each service that points at a 
common location and the deployment tools would need to configure that 
and create the shared directory. You don't save the deployment tools any 
effort this way.

Keeping a single lock path for everything running in a given service 
also eliminates the possibility that a lock gets used both in and out of 
os-brick, which would break if os-brick had a separate path. I don't 
know if that happens, but a single lock path means you never have to 
answer that question.

One drawback I can see to a single lock path for multiple services is 
possible collisions of lock names. Previously each service could name 
locks whatever they wanted as long as there were no duplicates within 
the service. If they start sharing lock path, there can be no duplicates 
between any of the services sharing the lock path. I can't say how much 
of a problem this would be, but we could probably come up with some way 
to look at the lock names used in a service and make sure there's no 

Also, I _think_ even if there were a duplicate lock name in two 
services, the worst that would happen is some unnecessary lock 
contention.* You might introduce a performance problem, but not a race. 
Personally, that's a tradeoff I'd happily make.

*: If you had multiple duplicate locks being acquired in different 
orders between the two services, it's possible you could introduce a 
deadlock. A lot has to go wrong for that to happen, but in the interest 
of completeness it should be considered. Have I mentioned that 
concurrency is hard? ;-)

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