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Date:   Mon, 29 Jun 2020 13:51:32 -0700
From:   Andre Guedes <andre.guedes@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, nhorman@...hat.com,
        sassmann@...hat.com, Aaron Brown <aaron.f.brown@...el.com>
Subject: Re: [net-next 05/13] igc: Check __IGC_PTP_TX_IN_PROGRESS instead of ptp_tx_skb

Hi Jakub,

Quoting Jakub Kicinski (2020-06-26 21:30:35)
> On Fri, 26 Jun 2020 18:54:23 -0700 Jeff Kirsher wrote:
> > From: Andre Guedes <andre.guedes@...el.com>
> > 
> > The __IGC_PTP_TX_IN_PROGRESS flag indicates we have a pending Tx
> > timestamp. In some places, instead of checking that flag, we check
> > adapter->ptp_tx_skb. This patch fixes those places to use the flag.
> > 
> > Quick note about igc_ptp_tx_hwtstamp() change: when that function is
> > called, adapter->ptp_tx_skb is expected to be valid always so we
> > WARN_ON_ONCE() in case it is not.
> > 
> > Quick note about igc_ptp_suspend() change: when suspending, we don't
> > really need to check if there is a pending timestamp. We can simply
> > clear it unconditionally.
> > 
> > Signed-off-by: Andre Guedes <andre.guedes@...el.com>
> > Tested-by: Aaron Brown <aaron.f.brown@...el.com>
> > Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
> > ---
> >  drivers/net/ethernet/intel/igc/igc_ptp.c | 16 +++++++---------
> >  1 file changed, 7 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/igc/igc_ptp.c b/drivers/net/ethernet/intel/igc/igc_ptp.c
> > index b1b23c6bf689..e65fdcf966b2 100644
> > --- a/drivers/net/ethernet/intel/igc/igc_ptp.c
> > +++ b/drivers/net/ethernet/intel/igc/igc_ptp.c
> > @@ -404,9 +404,6 @@ void igc_ptp_tx_hang(struct igc_adapter *adapter)
> >       bool timeout = time_is_before_jiffies(adapter->ptp_tx_start +
> >                                             IGC_PTP_TX_TIMEOUT);
> >  
> > -     if (!adapter->ptp_tx_skb)
> > -             return;
> > -
> >       if (!test_bit(__IGC_PTP_TX_IN_PROGRESS, &adapter->state))
> >               return;
> >  
> > @@ -435,6 +432,9 @@ static void igc_ptp_tx_hwtstamp(struct igc_adapter *adapter)
> >       struct igc_hw *hw = &adapter->hw;
> >       u64 regval;
> >  
> > +     if (WARN_ON_ONCE(!skb))
> > +             return;
> > +
> >       regval = rd32(IGC_TXSTMPL);
> >       regval |= (u64)rd32(IGC_TXSTMPH) << 32;
> >       igc_ptp_systim_to_hwtstamp(adapter, &shhwtstamps, regval);
> > @@ -466,7 +466,7 @@ static void igc_ptp_tx_work(struct work_struct *work)
> >       struct igc_hw *hw = &adapter->hw;
> >       u32 tsynctxctl;
> >  
> > -     if (!adapter->ptp_tx_skb)
> > +     if (!test_bit(__IGC_PTP_TX_IN_PROGRESS, &adapter->state))
> >               return;
> 
> Not that reading ptp_tx_skb is particularly correct here, but I think
> it's better. See how they get set:
> 
>                 if (adapter->tstamp_config.tx_type == HWTSTAMP_TX_ON &&
>                     !test_and_set_bit_lock(__IGC_PTP_TX_IN_PROGRESS,
>                                            &adapter->state)) {
>                         skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
>                         tx_flags |= IGC_TX_FLAGS_TSTAMP;
> 
>                         adapter->ptp_tx_skb = skb_get(skb);
>                         adapter->ptp_tx_start = jiffies;
> 
> bit is set first and other fields after. Since there is no locking here
> we may just see the bit but none of the fields set.

I see your point, but note that the code within the if-block and the code in
igc_ptp_tx_work() don't execute concurrently. adapter->ptp_tx_work is scheduled
only on a time-sync interrupt, which is triggered if IGC_TX_FLAGS_TSTAMP is
set (so adapter->ptp_tx_skb is valid).

- Andre

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