netdev - RE: [PATCH net] net: fec: avoid lock evasion when reading pps

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Message-ID:
 <PAXPR04MB85100C1A06253C0AE1EB36C288E22@PAXPR04MB8510.eurprd04.prod.outlook.com>
Date: Mon, 13 May 2024 07:53:38 +0000
From: Wei Fang <wei.fang@....com>
To: Eric Dumazet <edumazet@...gle.com>
CC: "davem@...emloft.net" <davem@...emloft.net>, "kuba@...nel.org"
	<kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>, Shenwei Wang
	<shenwei.wang@....com>, Clark Wang <xiaoning.wang@....com>,
	"richardcochran@...il.com" <richardcochran@...il.com>, "andrew@...n.ch"
	<andrew@...n.ch>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>
Subject: RE: [PATCH net] net: fec: avoid lock evasion when reading pps_enable

> -----Original Message-----
> From: Eric Dumazet <edumazet@...gle.com>
> Sent: 2024年5月13日 15:29
> To: Wei Fang <wei.fang@....com>
> Cc: davem@...emloft.net; kuba@...nel.org; pabeni@...hat.com; Shenwei
> Wang <shenwei.wang@....com>; Clark Wang <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 ?
For fec driver, the pps_channel and the reload_period of PPS request
are always fixed, and they were also not protected by the lock in the
original code.

> 
> 
> >
> >         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
> >