[v1] net: fec: avoid lock evasion when reading pps_enable

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

Posted by Wei Fang 1 year, 7 months ago

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

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

Posted by Simon Horman 1 year, 7 months ago

On Mon, May 13, 2024 at 09:51:26AM +0800, Wei Fang 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@nxp.com>

Reviewed-by: Simon Horman <horms@kernel.org>

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

Posted by Simon Horman 1 year, 7 months ago

On Mon, May 13, 2024 at 08:54:59PM +0100, Simon Horman wrote:
> On Mon, May 13, 2024 at 09:51:26AM +0800, Wei Fang 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@nxp.com>
> 
> Reviewed-by: Simon Horman <horms@kernel.org>

Sorry, I convinced myself this is correct.
But I now see that questions have been raised by others.
So please ignore the above.

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

Posted by Eric Dumazet 1 year, 7 months ago

On Mon, May 13, 2024 at 4:02 AM Wei Fang <wei.fang@nxp.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@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 == 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
>