Re: [dpdk-dev] [RFC 0/5] Use correct memory ordering in eal functions

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 24 Feb 2021 15:20:13 -0600
Honnappa Nagarahalli <honnappa.nagarahalli@arm.com> wrote:

> rte_eal_remote_launch and rte_eal_wait_lcore need to provide
> correct memory ordering to address the data communication from
> main core to worker core and vice versa.
> 
> There are 2 use cases:
> 1) All the store operations (meant for worker) by main core
> should be visible to worker core before the worker core runs the
> assigned function
> 
> 2) All the store operations by the worker core should be visible
> to the main core after rte_eal_wait_lcore returns.
> 
> For the data that needs to be communicated to the worker after
> the rte_eal_remote_launch returns, load-acquire and store-release
> semantics should be used (just like any other writer-reader use case).
> 
> For the main to worker communication, the pointer to function
> to execute is used as the guard variable. Hence, resetting of
> the function pointer is important.
> 
> For the worker to main communication, the existing code uses the
> lcore state as the guard variable. However, it looks like
> the FINISHED state is not really required. Hence the FINISHED state
> is removed before using the state as the guard variable. I would like
> some feedback on why the FINISHED state is required. I have not
> paid attention to what it means for backward compatibility.
> If it is decided to remove this state, documentation changes are
> required.
> 


Most likely all use of volatile in DPDK is suspect.
Perhaps we should re-enable the "volatile considered harmful" warning
in checkpatch