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Date: Thu, 1 May 2025 16:16:16 +0100
From: Simon Horman <horms@kernel.org>
To: Brian Vazquez <brianvv@google.com>
Cc: Brian Vazquez <brianvv.kernel@gmail.com>,
	Tony Nguyen <anthony.l.nguyen@intel.com>,
	Przemek Kitszel <przemyslaw.kitszel@intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	intel-wired-lan@lists.osuosl.org,
	David Decotigny <decot@google.com>,
	Anjali Singhai <anjali.singhai@intel.com>,
	Sridhar Samudrala <sridhar.samudrala@intel.com>,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	emil.s.tantilov@intel.com, Josh Hay <joshua.a.hay@intel.com>,
	Luigi Rizzo <lrizzo@google.com>
Subject: Re: [iwl-net PATCH v2] idpf: fix a race in txq wakeup
Message-ID: <20250501151616.GA3339421@horms.kernel.org>
References: <20250428195532.1590892-1-brianvv@google.com>
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In-Reply-To: <20250428195532.1590892-1-brianvv@google.com>

On Mon, Apr 28, 2025 at 07:55:32PM +0000, Brian Vazquez wrote:
> Add a helper function to correctly handle the lockless
> synchronization when the sender needs to block. The paradigm is
> 
>         if (no_resources()) {
>                 stop_queue();
>                 barrier();
>                 if (!no_resources())
>                         restart_queue();
>         }
> 
> netif_subqueue_maybe_stop already handles the paradigm correctly, but
> the code split the check for resources in three parts, the first one
> (descriptors) followed the protocol, but the other two (completions and
> tx_buf) were only doing the first part and so race prone.
> 
> Luckily netif_subqueue_maybe_stop macro already allows you to use a
> function to evaluate the start/stop conditions so the fix only requires
> the right helper function to evaluate all the conditions at once.
> 
> The patch removes idpf_tx_maybe_stop_common since it's no longer needed
> and instead adjusts separately the conditions for singleq and splitq.
> 
> Note that idpf_rx_buf_hw_update doesn't need to check for resources
> since that will be covered in idpf_tx_splitq_frame.

Should the above read idpf_tx_buf_hw_update() rather than
idpf_rx_buf_hw_update()?

If so, I see that this is true when idpf_tx_buf_hw_update() is called from
idpf_tx_singleq_frame(). But is a check required in the case where
idpf_rx_buf_hw_update() is called by idpf_tx_singleq_map()?

> 
> To reproduce:
> 
> Reduce the threshold for pending completions to increase the chances of
> hitting this pause by changing your kernel:
> 
> drivers/net/ethernet/intel/idpf/idpf_txrx.h
> 
> -#define IDPF_TX_COMPLQ_OVERFLOW_THRESH(txcq)   ((txcq)->desc_count >> 1)
> +#define IDPF_TX_COMPLQ_OVERFLOW_THRESH(txcq)   ((txcq)->desc_count >> 4)
> 
> Use pktgen to force the host to push small pkts very aggressively:
> 
> ./pktgen_sample02_multiqueue.sh -i eth1 -s 100 -6 -d $IP -m $MAC \
>   -p 10000-10000 -t 16 -n 0 -v -x -c 64
> 
> Fixes: 6818c4d5b3c2 ("idpf: add splitq start_xmit")
> Signed-off-by: Josh Hay <joshua.a.hay@intel.com>
> Signed-off-by: Brian Vazquez <brianvv@google.com>
> Signed-off-by: Luigi Rizzo <lrizzo@google.com>

...