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Date: Sat, 1 Apr 2023 17:18:12 +0200
From: Heiner Kallweit <hkallweit1@...il.com>
To: Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net
Cc: netdev@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com
Subject: Re: [PATCH net-next 1/3] net: provide macros for commonly copied
lockless queue stop/wake code
On 01.04.2023 07:12, Jakub Kicinski wrote:
> A lot of drivers follow the same scheme to stop / start queues
> without introducing locks between xmit and NAPI tx completions.
> I'm guessing they all copy'n'paste each other's code.
>
> Smaller drivers shy away from the scheme and introduce a lock
> which may cause deadlocks in netpoll.
>
> Provide macros which encapsulate the necessary logic.
>
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
> v2:
> - really flip the unlikely into a likely in __netif_tx_queue_maybe_wake()
> - convert if / else into pre-init of _ret
> v1: https://lore.kernel.org/all/20230322233028.269410-1-kuba@kernel.org/
> - perdicate -> predicate
> - on race use start instead of wake and make a note of that
> in the doc / comment at the start
> rfc: https://lore.kernel.org/all/20230311050130.115138-1-kuba@kernel.org/
> ---
> include/net/netdev_queues.h | 167 ++++++++++++++++++++++++++++++++++++
> 1 file changed, 167 insertions(+)
> create mode 100644 include/net/netdev_queues.h
>
> diff --git a/include/net/netdev_queues.h b/include/net/netdev_queues.h
> new file mode 100644
> index 000000000000..d050eb5e5bea
> --- /dev/null
> +++ b/include/net/netdev_queues.h
> @@ -0,0 +1,167 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_NET_QUEUES_H
> +#define _LINUX_NET_QUEUES_H
> +
> +#include <linux/netdevice.h>
> +
> +/* Lockless queue stopping / waking helpers.
> + *
> + * These macros are designed to safely implement stopping and waking
> + * netdev queues without full lock protection. We assume that there can
> + * be no concurrent stop attempts and no concurrent wake attempts.
> + * The try-stop should happen from the xmit handler*, while wake up
> + * should be triggered from NAPI poll context. The two may run
> + * concurrently (single producer, single consumer).
> + *
> + * All descriptor ring indexes (and other relevant shared state) must
> + * be updated before invoking the macros.
> + *
> + * * the try-stop side does not reschedule Tx (netif_tx_start_queue()
> + * instead of netif_tx_wake_queue()) so uses outside of the xmit
> + * handler may lead to bugs
> + */
> +
> +#define netif_tx_queue_try_stop(txq, get_desc, start_thrs) \
> + ({ \
> + int _res; \
> + \
> + netif_tx_stop_queue(txq); \
> + \
> + smp_mb(); \
> + \
> + /* We need to check again in a case another \
> + * CPU has just made room available. \
> + */ \
> + _res = 0; \
> + if (unlikely(get_desc >= start_thrs)) { \
> + netif_tx_start_queue(txq); \
> + _res = -1; \
> + } \
> + _res; \
> + }) \
> +
> +/**
> + * netif_tx_queue_maybe_stop() - locklessly stop a Tx queue, if needed
> + * @txq: struct netdev_queue to stop/start
> + * @get_desc: get current number of free descriptors (see requirements below!)
> + * @stop_thrs: minimal number of available descriptors for queue to be left
> + * enabled
> + * @start_thrs: minimal number of descriptors to re-enable the queue, can be
> + * equal to @stop_thrs or higher to avoid frequent waking
> + *
> + * All arguments may be evaluated multiple times, beware of side effects.
> + * @get_desc must be a formula or a function call, it must always
> + * return up-to-date information when evaluated!
> + * Expected to be used from ndo_start_xmit, see the comment on top of the file.
> + *
> + * Returns:
> + * 0 if the queue was stopped
> + * 1 if the queue was left enabled
> + * -1 if the queue was re-enabled (raced with waking)
> + */
> +#define netif_tx_queue_maybe_stop(txq, get_desc, stop_thrs, start_thrs) \
> + ({ \
> + int _res; \
> + \
> + _res = 1; \
> + if (unlikely(get_desc < stop_thrs)) \
> + _res = netif_tx_queue_try_stop(txq, get_desc, \
> + start_thrs); \
> + _res; \
> + }) \
> +
> +#define __netif_tx_queue_try_wake(txq, get_desc, start_thrs, down_cond) \
> + ({ \
> + int _res; \
> + \
> + /* Make sure that anybody stopping the queue after \
> + * this sees the new next_to_clean. \
> + */ \
> + smp_mb(); \
> + _res = 1; \
> + if (unlikely(netif_tx_queue_stopped(txq)) && !(down_cond)) { \
> + netif_tx_wake_queue(txq); \
> + _res = 0; \
> + } \
> + _res; \
> + })
> +
> +#define netif_tx_queue_try_wake(txq, get_desc, start_thrs) \
> + __netif_tx_queue_try_wake(txq, get_desc, start_thrs, false)
> +
> +/**
> + * __netif_tx_queue_maybe_wake() - locklessly wake a Tx queue, if needed
> + * @txq: struct netdev_queue to stop/start
> + * @get_desc: get current number of free descriptors (see requirements below!)
> + * @start_thrs: minimal number of descriptors to re-enable the queue
> + * @down_cond: down condition, predicate indicating that the queue should
> + * not be woken up even if descriptors are available
> + *
> + * All arguments may be evaluated multiple times.
> + * @get_desc must be a formula or a function call, it must always
> + * return up-to-date information when evaluated!
> + *
> + * Returns:
> + * 0 if the queue was woken up
> + * 1 if the queue was already enabled (or disabled but @down_cond is true)
> + * -1 if the queue was left stopped
> + */
> +#define __netif_tx_queue_maybe_wake(txq, get_desc, start_thrs, down_cond) \
> + ({ \
> + int _res; \
> + \
> + _res = -1; \
One more question: Don't we need a read memory barrier here to ensure
get_desc is up-to-date?
> + if (likely(get_desc > start_thrs)) \
> + _res = __netif_tx_queue_try_wake(txq, get_desc, \
> + start_thrs, \
> + down_cond); \
> + _res; \
> + })
> +
> +#define netif_tx_queue_maybe_wake(txq, get_desc, start_thrs) \
> + __netif_tx_queue_maybe_wake(txq, get_desc, start_thrs, false)
> +
> +/* subqueue variants follow */
> +
> +#define netif_subqueue_try_stop(dev, idx, get_desc, start_thrs) \
> + ({ \
> + struct netdev_queue *txq; \
> + \
> + txq = netdev_get_tx_queue(dev, idx); \
> + netif_tx_queue_try_stop(txq, get_desc, start_thrs); \
> + })
> +
> +#define netif_subqueue_maybe_stop(dev, idx, get_desc, stop_thrs, start_thrs) \
> + ({ \
> + struct netdev_queue *txq; \
> + \
> + txq = netdev_get_tx_queue(dev, idx); \
> + netif_tx_queue_maybe_stop(txq, get_desc, \
> + stop_thrs, start_thrs); \
> + })
> +
> +#define __netif_subqueue_try_wake(dev, idx, get_desc, start_thrs, down_cond) \
> + ({ \
> + struct netdev_queue *txq; \
> + \
> + txq = netdev_get_tx_queue(dev, idx); \
> + __netif_tx_queue_try_wake(txq, get_desc, \
> + start_thrs, down_cond); \
> + })
> +
> +#define netif_subqueue_try_wake(dev, idx, get_desc, start_thrs) \
> + __netif_subqueue_try_wake(dev, idx, get_desc, start_thrs, false)
> +
> +#define __netif_subqueue_maybe_wake(dev, idx, get_desc, start_thrs, down_cond) \
> + ({ \
> + struct netdev_queue *txq; \
> + \
> + txq = netdev_get_tx_queue(dev, idx); \
> + __netif_tx_queue_maybe_wake(txq, get_desc, \
> + start_thrs, down_cond); \
> + })
> +
> +#define netif_subqueue_maybe_wake(dev, idx, get_desc, start_thrs) \
> + __netif_subqueue_maybe_wake(dev, idx, get_desc, start_thrs, false)
> +
> +#endif
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