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

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Date: Mon, 13 May 2024 12:24:49 +0000
From: Wei Fang <wei.fang@....com>
To: Hariprasad Kelam <hkelam@...vell.com>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"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>
CC: "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: Hariprasad Kelam <hkelam@...vell.com>
> Sent: 2024年5月13日 17:27
> To: Wei Fang <wei.fang@....com>; davem@...emloft.net;
> edumazet@...gle.com; 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
> Cc: linux-kernel@...r.kernel.org; imx@...ts.linux.dev
> Subject: [PATCH net] net: fec: avoid lock evasion when reading pps_enable
> 
> See inline,
> 
> > 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;
> >
> >  	spin_lock_irqsave(&fep->tmreg_lock, flags);
> >
> > +	if (fep->pps_enable == enable) {
> 
> Can we atomic_set/get instead of spin_lock here.
> 
I'm afraid that cannot eliminate the lock evasion warning, because
it's still possible that multithreads take the false branch of
"if (fep->pps_enable == enable)" before pps_enable is updated.


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