linux-kernel - RE: [Intel-wired-lan] [PATCH iwl-net v1] igc: fix race condition in TX timestamp read for register 0

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Message-ID: <IA3PR11MB89860D4DD0D79426AE6C7447E511A@IA3PR11MB8986.namprd11.prod.outlook.com>
Date: Fri, 19 Sep 2025 19:34:54 +0000
From: "Loktionov, Aleksandr" <aleksandr.loktionov@...el.com>
To: "Choong, Chwee Lin" <chwee.lin.choong@...el.com>, "Nguyen, Anthony L"
	<anthony.l.nguyen@...el.com>, "Kitszel, Przemyslaw"
	<przemyslaw.kitszel@...el.com>, Andrew Lunn <andrew+netdev@...n.ch>, "David S
 . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, "Jakub
 Kicinski" <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Richard Cochran
	<richardcochran@...il.com>, "Gomes, Vinicius" <vinicius.gomes@...el.com>
CC: "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Shalev, Avi"
	<avi.shalev@...el.com>, "Song, Yoong Siang" <yoong.siang.song@...el.com>
Subject: RE: [Intel-wired-lan] [PATCH iwl-net v1] igc: fix race condition in
 TX timestamp read for register 0



> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@...osl.org> On Behalf
> Of Chwee-Lin Choong
> Sent: Thursday, September 18, 2025 8:38 PM
> To: Nguyen, Anthony L <anthony.l.nguyen@...el.com>; Kitszel,
> Przemyslaw <przemyslaw.kitszel@...el.com>; Andrew Lunn
> <andrew+netdev@...n.ch>; David S . Miller <davem@...emloft.net>; Eric
> Dumazet <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>; Paolo
> Abeni <pabeni@...hat.com>; Richard Cochran <richardcochran@...il.com>;
> Gomes, Vinicius <vinicius.gomes@...el.com>
> Cc: intel-wired-lan@...ts.osuosl.org; netdev@...r.kernel.org; linux-
> kernel@...r.kernel.org; Shalev, Avi <avi.shalev@...el.com>; Song,
> Yoong Siang <yoong.siang.song@...el.com>
> Subject: [Intel-wired-lan] [PATCH iwl-net v1] igc: fix race condition
> in TX timestamp read for register 0
> 
> The current HW bug workaround checks the TXTT_0 ready bit first, then
> reads LOW -> HIGH -> LOW from register 0 to detect if a timestamp was
> captured.
> 
> This sequence has a race: if a new timestamp is latched after reading
> the TXTT mask but before the first LOW read, both old and new
> timestamp match, causing the driver to drop a valid timestamp.
> 
> Fix by reading the LOW register first, then the TXTT mask, so a newly
> latched timestamp will always be detected.
> 
> This fix also prevents TX unit hangs observed under heavy timestamping
> load.
> 
> Fixes: c789ad7cbebc ("igc: Work around HW bug causing missing
> timestamps")
> Suggested-by: Avi Shalev <avi.shalev@...el.com>
> Signed-off-by: Song Yoong Siang <yoong.siang.song@...el.com>
> Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@...el.com>
> ---
>  drivers/net/ethernet/intel/igc/igc_ptp.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/igc/igc_ptp.c
> b/drivers/net/ethernet/intel/igc/igc_ptp.c
> index b7b46d863bee..930486b02fc1 100644
> --- a/drivers/net/ethernet/intel/igc/igc_ptp.c
> +++ b/drivers/net/ethernet/intel/igc/igc_ptp.c
> @@ -774,10 +774,17 @@ static void igc_ptp_tx_reg_to_stamp(struct
> igc_adapter *adapter,  static void igc_ptp_tx_hwtstamp(struct
> igc_adapter *adapter)  {
>  	struct igc_hw *hw = &adapter->hw;
> +	u32 txstmpl_old;
>  	u64 regval;
>  	u32 mask;
>  	int i;
> 
> +	/* Read the "low" register 0 first to establish a baseline
> value.
> +	 * This avoids a race where a new timestamp could be latched
> +	 * after checking the TXTT mask.
> +	 */
> +	txstmpl_old = rd32(IGC_TXSTMPL);
> +
>  	mask = rd32(IGC_TSYNCTXCTL) & IGC_TSYNCTXCTL_TXTT_ANY;
>  	if (mask & IGC_TSYNCTXCTL_TXTT_0) {
>  		regval = rd32(IGC_TXSTMPL);
> @@ -801,9 +808,8 @@ static void igc_ptp_tx_hwtstamp(struct igc_adapter
> *adapter)
>  		 * timestamp was captured, we can read the "high"
>  		 * register again.
>  		 */
> -		u32 txstmpl_old, txstmpl_new;
> +		u32 txstmpl_new;
> 
> -		txstmpl_old = rd32(IGC_TXSTMPL);
>  		rd32(IGC_TXSTMPH);
>  		txstmpl_new = rd32(IGC_TXSTMPL);
> 
> --
> 2.42.0

Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@...el.com>