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

[Thomas Monjalon <thomas@monjalon.net>]

25/04/2022 10:30, Thomas Monjalon:
> 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.

No comment after a month.
The patch is applied in next-net with the suggested rewording.