netdev - Re: [net-next PATCH v2 4/4] netdev: use napi_schedule bool instead of napi_schedule_prep/__napi

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Date: Sun, 8 Oct 2023 09:08:41 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Christian Marangi <ansuelsmth@...il.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>, 
	Wolfgang Grandegger <wg@...ndegger.com>, Marc Kleine-Budde <mkl@...gutronix.de>, 
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Subject: Re: [net-next PATCH v2 4/4] netdev: use napi_schedule bool instead of napi_schedule_prep/__napi_schedule

On Fri, Oct 6, 2023 at 8:49 PM Christian Marangi <ansuelsmth@...il.com> wrote:
>
> On Thu, Oct 05, 2023 at 06:16:26PM +0200, Eric Dumazet wrote:
> > On Tue, Oct 3, 2023 at 8:36 PM Christian Marangi <ansuelsmth@...il.com> wrote:
> > >
> > > Replace if condition of napi_schedule_prep/__napi_schedule and use bool
> > > from napi_schedule directly where possible.
> > >
> > > Signed-off-by: Christian Marangi <ansuelsmth@...il.com>
> > > ---
> > >  drivers/net/ethernet/atheros/atlx/atl1.c     | 4 +---
> > >  drivers/net/ethernet/toshiba/tc35815.c       | 4 +---
> > >  drivers/net/wireless/intel/iwlwifi/pcie/rx.c | 4 +---
> > >  3 files changed, 3 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/drivers/net/ethernet/atheros/atlx/atl1.c b/drivers/net/ethernet/atheros/atlx/atl1.c
> > > index 02aa6fd8ebc2..a9014d7932db 100644
> > > --- a/drivers/net/ethernet/atheros/atlx/atl1.c
> > > +++ b/drivers/net/ethernet/atheros/atlx/atl1.c
> > > @@ -2446,7 +2446,7 @@ static int atl1_rings_clean(struct napi_struct *napi, int budget)
> > >
> > >  static inline int atl1_sched_rings_clean(struct atl1_adapter* adapter)
> > >  {
> > > -       if (!napi_schedule_prep(&adapter->napi))
> > > +       if (!napi_schedule(&adapter->napi))
> > >                 /* It is possible in case even the RX/TX ints are disabled via IMR
> > >                  * register the ISR bits are set anyway (but do not produce IRQ).
> > >                  * To handle such situation the napi functions used to check is
> > > @@ -2454,8 +2454,6 @@ static inline int atl1_sched_rings_clean(struct atl1_adapter* adapter)
> > >                  */
> > >                 return 0;
> > >
> > > -       __napi_schedule(&adapter->napi);
> > > -
> > >         /*
> > >          * Disable RX/TX ints via IMR register if it is
> > >          * allowed. NAPI handler must reenable them in same
> > > diff --git a/drivers/net/ethernet/toshiba/tc35815.c b/drivers/net/ethernet/toshiba/tc35815.c
> > > index 14cf6ecf6d0d..a8b8a0e13f9a 100644
> > > --- a/drivers/net/ethernet/toshiba/tc35815.c
> > > +++ b/drivers/net/ethernet/toshiba/tc35815.c
> > > @@ -1436,9 +1436,7 @@ static irqreturn_t tc35815_interrupt(int irq, void *dev_id)
> > >         if (!(dmactl & DMA_IntMask)) {
> > >                 /* disable interrupts */
> > >                 tc_writel(dmactl | DMA_IntMask, &tr->DMA_Ctl);
> > > -               if (napi_schedule_prep(&lp->napi))
> > > -                       __napi_schedule(&lp->napi);
> > > -               else {
> > > +               if (!napi_schedule(&lp->napi)) {
> > >                         printk(KERN_ERR "%s: interrupt taken in poll\n",
> > >                                dev->name);
> > >                         BUG();
> >
> > Hmmm... could you also remove this BUG() ? I think this code path can be taken
> > if some applications are using busy polling.
> >
> > Or simply rewrite this with the traditional
> >
> > if (napi_schedule_prep(&lp->napi)) {
> >    /* disable interrupts */
> >    tc_writel(dmactl | DMA_IntMask, &tr->DMA_Ctl);
> >     __napi_schedule(&lp->napi);
> > }
> >
> >
>
> Mhhh is it safe to do so? I mean it seems very wrong to print a warning
> and BUG() instead of disabling the interrupt only if napi can be
> scheduled... Maybe is very old code? The more I see this the more I see
> problem... (randomly disabling the interrupt and then make the kernel
> die)

I am pretty sure this BUG() can be hit these days with busy polling or
setting gro_flush_timeout.

I wish we could remove these bugs before someone copy-paste them.

Again, this is orthogonal, I might simply stop doing reviews if this
is not useful.