RE: [PATCH net] net: fec: avoid lock evasion when reading pps_enable

[Wei Fang]

> -----Original Message-----
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> Sent: 2024年5月13日 16:41
> To: Wei Fang <wei.fang@xxxxxxx>
> Cc: davem@xxxxxxxxxxxxx; kuba@xxxxxxxxxx; pabeni@xxxxxxxxxx; Shenwei
> Wang <shenwei.wang@xxxxxxx>; Clark Wang <xiaoning.wang@xxxxxxx>;
> richardcochran@xxxxxxxxx; andrew@xxxxxxx; netdev@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; imx@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH net] net: fec: avoid lock evasion when reading
> pps_enable
> 
> On Mon, May 13, 2024 at 9:53 AM Wei Fang <wei.fang@xxxxxxx> wrote:
> >
> > > -----Original Message-----
> > > From: Eric Dumazet <edumazet@xxxxxxxxxx>
> > > Sent: 2024年5月13日 15:29
> > > To: Wei Fang <wei.fang@xxxxxxx>
> > > Cc: davem@xxxxxxxxxxxxx; kuba@xxxxxxxxxx; pabeni@xxxxxxxxxx;
> Shenwei
> > > Wang <shenwei.wang@xxxxxxx>; Clark Wang
> <xiaoning.wang@xxxxxxx>;
> > > richardcochran@xxxxxxxxx; andrew@xxxxxxx; netdev@xxxxxxxxxxxxxxx;
> > > linux-kernel@xxxxxxxxxxxxxxx; imx@xxxxxxxxxxxxxxx
> > > 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@xxxxxxx>
> 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@xxxxxxx>
> > > > ---
> > > >  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.
> 
> If this is the case, please move this initialization elsewhere, so that we can be
> absolutely sure of the  claim.
> 
Accept, thanks


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