[PATCH] ethdev: recommend against using locks in event callbacks

[Kevin Traynor]

On 01/02/2024 08:43, David Marchand wrote:
> As described in a recent bugzilla opened against the net/iavf driver,
> a driver may call a event callback from other calls of the ethdev API.
> 
> Nothing guarantees in the ethdev API against such behavior.
> 
> Add a notice against using locks in those callbacks.
> 
> Bugzilla ID: 1337
> 
> Signed-off-by: David Marchand <david.marchand at redhat.com>
> ---
>  lib/ethdev/rte_ethdev.h | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
> index 21e3a21903..5c6b104fb4 100644
> --- a/lib/ethdev/rte_ethdev.h
> +++ b/lib/ethdev/rte_ethdev.h
> @@ -4090,7 +4090,19 @@ enum rte_eth_event_type {
>  	RTE_ETH_EVENT_MAX       /**< max value of this enum */
>  };
>  
> -/** User application callback to be registered for interrupts. */
> +/**
> + * User application callback to be registered for interrupts.
> + *
> + * Note: there is no guarantee in the DPDK drivers that a callback won't be
> + *       called in the middle of other parts of the ethdev API. For example,
> + *       imagine that thread A calls rte_eth_dev_start() and as part of this
> + *       call, a RTE_ETH_EVENT_INTR_RESET event gets generated and the
> + *       associated callback is ran on thread A. In that example, if the
> + *       application protects its internal data using locks before calling
> + *       rte_eth_dev_start(), and the callback takes a same lock, a deadlock
> + *       occurs. Because of this, it is highly recommended NOT to take locks in
> + *       those callbacks.
> + */

That is a good practical recommendation for an application developer.

I wonder if it should taken further so that the API formally states the
callback MUST be non-blocking?

>  typedef int (*rte_eth_dev_cb_fn)(uint16_t port_id,
>  		enum rte_eth_event_type event, void *cb_arg, void *ret_param);
>