linux-kernel - Re: [iwl-net PATCH v2] idpf: fix a race in txq wakeup

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Message-ID: <CAMzD94SNJe3QcLgNCPtVqDa69B7qcghcBkSOPWzV43d_XAeYuQ@mail.gmail.com>
Date: Thu, 1 May 2025 12:51:48 -0400
From: Brian Vazquez <brianvv@...gle.com>
To: Simon Horman <horms@...nel.org>
Cc: Brian Vazquez <brianvv.kernel@...il.com>, Tony Nguyen <anthony.l.nguyen@...el.com>, 
	Przemek Kitszel <przemyslaw.kitszel@...el.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	intel-wired-lan@...ts.osuosl.org, David Decotigny <decot@...gle.com>, 
	Anjali Singhai <anjali.singhai@...el.com>, Sridhar Samudrala <sridhar.samudrala@...el.com>, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	emil.s.tantilov@...el.com, Josh Hay <joshua.a.hay@...el.com>, 
	Luigi Rizzo <lrizzo@...gle.com>
Subject: Re: [iwl-net PATCH v2] idpf: fix a race in txq wakeup

On Thu, May 1, 2025 at 11:16 AM Simon Horman <horms@...nel.org> wrote:
>
> 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()?

Nice catch, that's a typo indeed.

>
> 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()?

No, the check is not required. The call is at the end of
idpf_tx_singleq_map at that point you already checked for resources
and you're about to send the pkt.

>
> >
> > 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@...el.com>
> > Signed-off-by: Brian Vazquez <brianvv@...gle.com>
> > Signed-off-by: Luigi Rizzo <lrizzo@...gle.com>
>
> ...