Re: [PATCH] ethdev: prohibit polling of a stopped queue

[Thomas Monjalon <thomas@monjalon.net>]

10/04/2022 23:35, Dmitry Kozlyuk:
> Whether it is allowed to call Rx/Tx functions for a stopped queue
> was undocumented. Some PMDs make this behavior a no-op
> either by explicitly checking the queue state
> or by the way how their routines are implemented or HW works.
> 
> No-op behavior may be convenient for application developers.
> But it also means that pollers of stopped queues
> would go all the way down to PMD Rx/Tx routines, wasting cycles.
> Some PMDs would do a check for the queue state on data path,
> even though it may never be needed for a particular application.
> Also, use cases for stopping queues or starting them deferred
> do not logically require polling stopped queues.
> 
> Use case 1: a secondary that was polling the queue has crashed,
> the primary is doing a recovery to free all mbufs.
> By definition the queue to be restarted is not polled.
> 
> Use case 2: deferred queue start or queue reconfiguration.
> The polling thread must be synchronized anyway,
> because queue start and stop are non-atomic.
> 
> Prohibit calling Rx/Tx functions on stopped queues.
> 
> Fixes: 0748be2cf9a2 ("ethdev: queue start and stop")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
> ---
> This patch is was originally a part of the series:
> http://patchwork.dpdk.org/project/dpdk/patch/20220307125351.697936-3-dkozlyuk@nvidia.com/
> The discussion there is summarized in the commit message.
[...]
>   * rte_eth_rx_queue_setup()), it must call rte_eth_dev_stop() first to stop the
>   * device and then do the reconfiguration before calling rte_eth_dev_start()
>   * again. The transmit and receive functions should not be invoked when the
> - * device is stopped.
> + * device is stopped or when the queue is stopped (for that queue).

I think we can make it simpler:

The transmit and receive functions should not be invoked when the device
or the queue is stopped.