On Sat, Sep 30, 2023 at 03:42:20PM +0200, Eric Dumazet wrote:
> On Sat, Sep 30, 2023 at 2:11 PM Christian Marangi <[email protected]> wrote:
> >
> > On Sat, Sep 30, 2023 at 01:59:53PM +0200, Eric Dumazet wrote:
> > > On Fri, Sep 22, 2023 at 1:13 PM Christian Marangi <[email protected]> wrote:
> > > >
> > > > We currently have napi_if_scheduled_mark_missed that can be used to
> > > > check if napi is scheduled but that does more thing than simply checking
> > > > it and return a bool. Some driver already implement custom function to
> > > > check if napi is scheduled.
> > > >
> > > > Drop these custom function and introduce napi_is_scheduled that simply
> > > > check if napi is scheduled atomically.
> > > >
> > > > Update any driver and code that implement a similar check and instead
> > > > use this new helper.
> > > >
> > > > Signed-off-by: Christian Marangi <[email protected]>
> > > > ---
> > > > drivers/net/ethernet/chelsio/cxgb3/sge.c | 8 --------
> > > > drivers/net/wireless/realtek/rtw89/core.c | 2 +-
> > > > include/linux/netdevice.h | 5 +++++
> > > > net/core/dev.c | 2 +-
> > > > 4 files changed, 7 insertions(+), 10 deletions(-)
> > > >
> > > > diff --git a/drivers/net/ethernet/chelsio/cxgb3/sge.c b/drivers/net/ethernet/chelsio/cxgb3/sge.c
> > > > index 2e9a74fe0970..71fa2dc19034 100644
> > > > --- a/drivers/net/ethernet/chelsio/cxgb3/sge.c
> > > > +++ b/drivers/net/ethernet/chelsio/cxgb3/sge.c
> > > > @@ -2501,14 +2501,6 @@ static int napi_rx_handler(struct napi_struct *napi, int budget)
> > > > return work_done;
> > > > }
> > > >
> > > > -/*
> > > > - * Returns true if the device is already scheduled for polling.
> > > > - */
> > > > -static inline int napi_is_scheduled(struct napi_struct *napi)
> > > > -{
> > > > - return test_bit(NAPI_STATE_SCHED, &napi->state);
> > > > -}
> > > > -
> > > > /**
> > > > * process_pure_responses - process pure responses from a response queue
> > > > * @adap: the adapter
> > > > diff --git a/drivers/net/wireless/realtek/rtw89/core.c b/drivers/net/wireless/realtek/rtw89/core.c
> > > > index 133bf289bacb..bbf4ea3639d4 100644
> > > > --- a/drivers/net/wireless/realtek/rtw89/core.c
> > > > +++ b/drivers/net/wireless/realtek/rtw89/core.c
> > > > @@ -1744,7 +1744,7 @@ static void rtw89_core_rx_to_mac80211(struct rtw89_dev *rtwdev,
> > > > struct napi_struct *napi = &rtwdev->napi;
> > > >
> > > > /* In low power mode, napi isn't scheduled. Receive it to netif. */
> > > > - if (unlikely(!test_bit(NAPI_STATE_SCHED, &napi->state)))
> > > > + if (unlikely(!napi_is_scheduled(napi)))
> > > > napi = NULL;
> > > >
> > > > rtw89_core_hw_to_sband_rate(rx_status);
> > > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > > index db3d8429d50d..8eac00cd3b92 100644
> > > > --- a/include/linux/netdevice.h
> > > > +++ b/include/linux/netdevice.h
> > > > @@ -482,6 +482,11 @@ static inline bool napi_prefer_busy_poll(struct napi_struct *n)
> > > > return test_bit(NAPI_STATE_PREFER_BUSY_POLL, &n->state);
> > > > }
> > > >
> > >
> > >
> > > In which context is it safe to call this helper ?
> > >
> >
> > test_bit is atomic so it should be always safe. Also the idea of this
> > check (and from what I can see this apply also to the other 2 user) is
> > somehow best effort, we check if in the current istant there is a napi
> > scheduled and we act.
>
> I think testing a bit here is not enough to take any kind of useful decision,
> unless used in a particular context.
>
Ehhh the idea here was to reduce code duplication since the very same
test will be done in stmmac. So I guess this code cleanup is a NACK and
I have to duplicate the test in the stmmac driver.
> >
> > > I fear that making this available will add more bugs.
> > >
> > > For instance rspq_check_napi() seems buggy to me.
> > >
> >
> > Mhhh why? Am I opening a can of worms?
>
> Yes I think :/
>
> Because only the thread that has grabbed the bit can make any sense of it.
>
> Another thread reading it would not really know if the value is not going to
> change immediately. So what would be the point ?
>
> It seems rspq_check_napi() real intent was maybe the following,
> but really this is hard to know if the current race in this code is okay or not.
>
> diff --git a/drivers/net/ethernet/chelsio/cxgb3/sge.c
> b/drivers/net/ethernet/chelsio/cxgb3/sge.c
> index 2e9a74fe0970df333226b80af8716f30865c01b7..e153c9590b36b38e430bc93660146b428e9b3347
> 100644
> --- a/drivers/net/ethernet/chelsio/cxgb3/sge.c
> +++ b/drivers/net/ethernet/chelsio/cxgb3/sge.c
> @@ -2676,8 +2676,10 @@ static int rspq_check_napi(struct sge_qset *qs)
>
> if (!napi_is_scheduled(&qs->napi) &&
> is_new_response(&q->desc[q->cidx], q)) {
> - napi_schedule(&qs->napi);
> - return 1;
> + if (napi_schedule_prep(&qs->napi)) {
> + __napi_schedule(&qs->napi);
> + return 1;
> + }
> }
> return 0;
> }
--
Ansuel
On Mon, Oct 2, 2023 at 2:29 PM Christian Marangi <[email protected]> wrote:
> Ehhh the idea here was to reduce code duplication since the very same
> test will be done in stmmac. So I guess this code cleanup is a NACK and
> I have to duplicate the test in the stmmac driver.
I simply wanted to add a comment in front of this function/helper,
advising not using it unless absolutely needed.
Thus my question "In which context is it safe to call this helper ?"
As long as it was private with a driver, I did not mind.
But if made public in include/linux/netdevice.h, I would rather not
have to explain
to future users why it can be problematic.
On Mon, Oct 02, 2023 at 02:35:22PM +0200, Eric Dumazet wrote:
> On Mon, Oct 2, 2023 at 2:29 PM Christian Marangi <[email protected]> wrote:
>
> > Ehhh the idea here was to reduce code duplication since the very same
> > test will be done in stmmac. So I guess this code cleanup is a NACK and
> > I have to duplicate the test in the stmmac driver.
>
> I simply wanted to add a comment in front of this function/helper,
> advising not using it unless absolutely needed.
>
> Thus my question "In which context is it safe to call this helper ?"
>
> As long as it was private with a driver, I did not mind.
>
> But if made public in include/linux/netdevice.h, I would rather not
> have to explain
> to future users why it can be problematic.
Oh ok!
We have plenty of case similar to this. (example some clock API very
internal that should not be used normally or regmap related)
I will include some comments warning that this should not be used in
normal circumstances and other warnings. If you have suggestion on what
to add feel free to write them.
Any clue on how to proceed with the sge driver?
--
Ansuel