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From: Eric Dumazet <edumazet@google.com>
Date: Mon, 13 May 2024 10:40:46 +0200
Message-ID: <CANn89i+0xtG5hg2LsGvNOueCAkOac_UccXWq-Eeqt1a40EkqBA@mail.gmail.com>
Subject: Re: [PATCH net] net: fec: avoid lock evasion when reading pps_enable
To: Wei Fang <wei.fang@nxp.com>
Cc: "davem@davemloft.net" <davem@davemloft.net>, "kuba@kernel.org" <kuba@kernel.org>, 
	"pabeni@redhat.com" <pabeni@redhat.com>, Shenwei Wang <shenwei.wang@nxp.com>, 
	Clark Wang <xiaoning.wang@nxp.com>, 
	"richardcochran@gmail.com" <richardcochran@gmail.com>, "andrew@lunn.ch" <andrew@lunn.ch>, 
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "imx@lists.linux.dev" <imx@lists.linux.dev>
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On Mon, May 13, 2024 at 9:53=E2=80=AFAM Wei Fang <wei.fang@nxp.com> wrote:
>
> > -----Original Message-----
> > From: Eric Dumazet <edumazet@google.com>
> > Sent: 2024=E5=B9=B45=E6=9C=8813=E6=97=A5 15:29
> > To: Wei Fang <wei.fang@nxp.com>
> > Cc: davem@davemloft.net; kuba@kernel.org; pabeni@redhat.com; Shenwei
> > Wang <shenwei.wang@nxp.com>; Clark Wang <xiaoning.wang@nxp.com>;
> > richardcochran@gmail.com; andrew@lunn.ch; netdev@vger.kernel.org;
> > linux-kernel@vger.kernel.org; imx@lists.linux.dev
> > Subject: Re: [PATCH net] net: fec: avoid lock evasion when reading pps_=
enable
> >
> > On Mon, May 13, 2024 at 4:02=E2=80=AFAM Wei Fang <wei.fang@nxp.com> wro=
te:
> > >
> > > 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 wa=
rnings.
> > >
> > > Fixes: 278d24047891 ("net: fec: ptp: Enable PPS output based on ptp
> > > clock")
> > > Signed-off-by: Wei Fang <wei.fang@nxp.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 =3D=3D enable)
> > > -               return 0;
> > > -
> > >         fep->pps_channel =3D DEFAULT_PPS_CHANNEL;
> > >         fep->reload_period =3D 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.

If this is the case, please move this initialization elsewhere, so
that we can be absolutely sure of the  claim.

I see fep->reload_period being overwritten in this file, I do not see
clear evidence this is all safe.