[lttng-dev] QSBR urcu read lock question

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Thu Apr 15 15:26:00 EDT 2021

----- On Apr 15, 2021, at 2:11 PM, lbj lbj137 at yahoo.com wrote:

> Thanks Mathieu. Is it safe to assume that if call_rcu is called twice then the
> callbacks are executed in the order that call_rcu was invoked? I think there is
> a queue and only one thread that QSBR uses to handle callbacks, i just wanted
> to make sure that the queue was a guaranteed fifo.

It may very well depend on your configuration. For instance, if an application
invokes create_all_cpu_call_rcu_data(), it will create per-cpu worker threads
on each possible CPU. In that configuration, a given thread invoking call_rcu()
twice (back to back) may be migrated from one CPU to another in between, hence
different call-rcu worker threads will be responsible for executing the callbacks,
and they can be executed in any order.

So even though by careful analysis of your specific application configuration you
may presume that the callbacks will be executed in the same order they were invoked,
this is not guaranteed by the API, so I would not recommend relying on FIFO order

And given that the call_rcu API does not guarantee FIFO order, we could eventually
decide to change the order of callback execution from FIFO to LIFO if it leads to
performance improvements due to better cache locality.



Mathieu Desnoyers
EfficiOS Inc.

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