[openstack-dev] [oslo][concurrency] lockutils lock fairness / starvation

Joshua Harlow harlowja at fastmail.com
Tue May 16 16:45:42 UTC 2017

So fyi,

If you really want something like this:

Just use:


And always get a write lock.

It is a slightly different way of getting those locks (via a context 
manager) but the implementation underneath is a deque; so fairness 
should be assured in FIFO order...





Chris Friesen wrote:
> On 05/15/2017 03:42 PM, Clint Byrum wrote:
>> In order to implement fairness you'll need every lock request to happen
>> in a FIFO queue. This is often implemented with a mutex-protected queue
>> of condition variables. Since the mutex for the queue is only held while
>> you append to the queue, you will always get the items from the queue
>> in the order they were written to it.
>> So you have lockers add themselves to the queue and wait on their
>> condition variable, and then a thread running all the time that reads
>> the queue and acts on each condition to make sure only one thread is
>> activated at a time (or that one thread can just always do all the work
>> if the arguments are simple enough to put in a queue).
> Do you even need the extra thread? The implementations I've seen for a
> ticket lock (in C at least) usually have the unlock routine wake up the
> next pending locker.
> Chris
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