linux-kernel - Re: [PATCH net] net: fec: avoid lock evasion when reading pps

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Date: Mon, 13 May 2024 09:29:28 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Wei Fang <wei.fang@....com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	shenwei.wang@....com, xiaoning.wang@....com, richardcochran@...il.com, 
	andrew@...n.ch, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	imx@...ts.linux.dev
Subject: Re: [PATCH net] net: fec: avoid lock evasion when reading pps_enable

On Mon, May 13, 2024 at 4:02 AM Wei Fang <wei.fang@....com> wrote:
>
> The assignment of pps_enable is protected by tmreg_lock, but the read
> operation of pps_enable is not. So the Coverity tool reports a lock
> evasion warning which may cause data race to occur when running in a
> multithread environment. Although this issue is almost impossible to
> occur, we'd better fix it, at least it seems more logically reasonable,
> and it also prevents Coverity from continuing to issue warnings.
>
> Fixes: 278d24047891 ("net: fec: ptp: Enable PPS output based on ptp clock")
> Signed-off-by: Wei Fang <wei.fang@....com>
> ---
>  drivers/net/ethernet/freescale/fec_ptp.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/ethernet/freescale/fec_ptp.c b/drivers/net/ethernet/freescale/fec_ptp.c
> index 181d9bfbee22..8d37274a3fb0 100644
> --- a/drivers/net/ethernet/freescale/fec_ptp.c
> +++ b/drivers/net/ethernet/freescale/fec_ptp.c
> @@ -104,14 +104,16 @@ static int fec_ptp_enable_pps(struct fec_enet_private *fep, uint enable)
>         struct timespec64 ts;
>         u64 ns;
>
> -       if (fep->pps_enable == enable)
> -               return 0;
> -
>         fep->pps_channel = DEFAULT_PPS_CHANNEL;
>         fep->reload_period = PPS_OUPUT_RELOAD_PERIOD;

Why are these writes left without the spinlock protection ?


>
>         spin_lock_irqsave(&fep->tmreg_lock, flags);
>
> +       if (fep->pps_enable == enable) {
> +               spin_unlock_irqrestore(&fep->tmreg_lock, flags);
> +               return 0;
> +       }
> +
>         if (enable) {
>                 /* clear capture or output compare interrupt status if have.
>                  */
> --
> 2.34.1
>