linux-kernel - Re: [Intel-wired-lan] [PATCH v2] idpf: increment completion queue next_to

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Message-ID: <CAODvEq4Fma_N+oRMuuW2X-BbnkSNUxbHiwh6dDA_3Q0YKR_mdw@mail.gmail.com>
Date: Sun, 4 Jan 2026 23:48:36 -0800
From: Li Li <boolli@...gle.com>
To: "Loktionov, Aleksandr" <aleksandr.loktionov@...el.com>
Cc: "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>, 
	"Kitszel, Przemyslaw" <przemyslaw.kitszel@...el.com>, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>, 
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>, 
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, David Decotigny <decot@...gle.com>, 
	"Singhai, Anjali" <anjali.singhai@...el.com>, 
	"Samudrala, Sridhar" <sridhar.samudrala@...el.com>, Brian Vazquez <brianvv@...gle.com>, 
	"Tantilov, Emil S" <emil.s.tantilov@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH v2] idpf: increment completion queue
 next_to_clean in sw marker wait routine

On Sun, Jan 4, 2026 at 11:43 PM Loktionov, Aleksandr
<aleksandr.loktionov@...el.com> wrote:
>
>
>
>
>
> From: Li Li <boolli@...gle.com>
> Sent: Monday, January 5, 2026 8:39 AM
> To: Loktionov, Aleksandr <aleksandr.loktionov@...el.com>
> Cc: Nguyen, Anthony L <anthony.l.nguyen@...el.com>; Kitszel, Przemyslaw <przemyslaw.kitszel@...el.com>; David S. Miller <davem@...emloft.net>; Jakub Kicinski <kuba@...nel.org>; Eric Dumazet <edumazet@...gle.com>; intel-wired-lan@...ts.osuosl.org; netdev@...r.kernel.org; linux-kernel@...r.kernel.org; David Decotigny <decot@...gle.com>; Singhai, Anjali <anjali.singhai@...el.com>; Samudrala, Sridhar <sridhar.samudrala@...el.com>; Brian Vazquez <brianvv@...gle.com>; Tantilov, Emil S <emil.s.tantilov@...el.com>
> Subject: Re: [Intel-wired-lan] [PATCH v2] idpf: increment completion queue next_to_clean in sw marker wait routine
>
>
>
>
>
>
>
> On Sun, Jan 4, 2026 at 11:19 PM Loktionov, Aleksandr <aleksandr.loktionov@...el.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Intel-wired-lan <intel-wired-lan-bounces@...osl.org> On Behalf
> > Of Li Li via Intel-wired-lan
> > Sent: Monday, January 5, 2026 7:47 AM
> > To: Nguyen, Anthony L <anthony.l.nguyen@...el.com>; Kitszel,
> > Przemyslaw <przemyslaw.kitszel@...el.com>; David S. Miller
> > <davem@...emloft.net>; Jakub Kicinski <kuba@...nel.org>; Eric Dumazet
> > <edumazet@...gle.com>; intel-wired-lan@...ts.osuosl.org
> > Cc: netdev@...r.kernel.org; linux-kernel@...r.kernel.org; David
> > Decotigny <decot@...gle.com>; Singhai, Anjali
> > <anjali.singhai@...el.com>; Samudrala, Sridhar
> > <sridhar.samudrala@...el.com>; Brian Vazquez <brianvv@...gle.com>;
> > Tantilov, Emil S <emil.s.tantilov@...el.com>; Li Li
> > <boolli@...gle.com>
> > Subject: [Intel-wired-lan] [PATCH v2] idpf: increment completion queue
> > next_to_clean in sw marker wait routine
> >
> > Currently, in idpf_wait_for_sw_marker_completion(), when an
> > IDPF_TXD_COMPLT_SW_MARKER packet is found, the routine breaks out of
> > the for loop and does not increment the next_to_clean counter. This
> > causes the subsequent NAPI polls to run into the same
> > IDPF_TXD_COMPLT_SW_MARKER packet again and print out the following:
> >
> >     [   23.261341] idpf 0000:05:00.0 eth1: Unknown TX completion type:
> > 5
> >
> > Instead, we should increment next_to_clean regardless when an
> > IDPF_TXD_COMPLT_SW_MARKER packet is found.
> >
> > Tested: with the patch applied, we do not see the errors above from
> > NAPI polls anymore.
> >
> > Signed-off-by: Li Li <boolli@...gle.com>
> > ---
> > Changes in v2:
> >  - Initialize idpf_tx_queue *target to NULL to suppress the "'target'
> >    uninitialized when 'if' statement is true warning".
> >
> >  drivers/net/ethernet/intel/idpf/idpf_txrx.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> > b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> > index 69bab7187e541..452d0a9e83a4f 100644
> > --- a/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> > +++ b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> > @@ -2326,7 +2326,7 @@ void idpf_wait_for_sw_marker_completion(const
> > struct idpf_tx_queue *txq)
> >
> >       do {
> >               struct idpf_splitq_4b_tx_compl_desc *tx_desc;
> > -             struct idpf_tx_queue *target;
> > +             struct idpf_tx_queue *target = NULL;
> Linux kernel is against premature initialization just to silence a compiler.
> The target variable is dereferenced at idpf_queue_clear(SW_MARKER, target))
> but can remain uninitialized if execution jumps to the next: label via a goto
> before target is assigned.
> Isn't it?
>
> That is correct. When the following if statement (line 2341-2343) evaluates to true:
>
>
>
>   if (FIELD_GET(IDPF_TXD_COMPLQ_COMPL_TYPE_M, ctype_gen) !=
>    IDPF_TXD_COMPLT_SW_MARKER)
>     goto next;
>
>
>
> Then the initialization at line 2346:
>
>
>
>   target = complq->txq_grp->txqs[id];
>
>
>
> would be skipped, making "target" uninitialized.
>
>
>
> Therefore, in this patch, I need to initialize "target" to NULL.
>
>
>
> The ‘NULL’ target variable can be dereferenced at idpf_queue_clear(SW_MARKER, target)), isn’t it?

That would not be possible, because right before
"idpf_queue_clear(SW_MARKER, target))", "target"
is initialized to "complq->txq_grp->txqs[id]":

  if (FIELD_GET(IDPF_TXD_COMPLQ_COMPL_TYPE_M, ctype_gen) !=
    IDPF_TXD_COMPLT_SW_MARKER)
    goto next;

  id = FIELD_GET(IDPF_TXD_COMPLQ_QID_M, ctype_gen);
  target = complq->txq_grp->txqs[id];

  idpf_queue_clear(SW_MARKER, target);

"target" only remains uninitialized if the if statement above
evaluates to true and skips the initialization.

>
>
>
>
>
>
> >               u32 ctype_gen, id;
> >
> >               tx_desc = flow ? &complq->comp[ntc].common :
> > @@ -2346,14 +2346,14 @@ void idpf_wait_for_sw_marker_completion(const
> > struct idpf_tx_queue *txq)
> >               target = complq->txq_grp->txqs[id];
> >
> >               idpf_queue_clear(SW_MARKER, target);
> > -             if (target == txq)
> > -                     break;
> >
> >  next:
> >               if (unlikely(++ntc == complq->desc_count)) {
> >                       ntc = 0;
> >                       gen_flag = !gen_flag;
> >               }
> > +             if (target == txq)
> Are tou sure that incremented ntc value is ever written back to complq->next_to_clean?
>
>
>
> Yes, the value of "ntc" is written back to "complq->next_to_clean" at the end of the function
>
>  (at line 2360):
>
>
>
>   complq->next_to_clean = ntc;
>
> Thank you, I don’t see it from the patch.
>
>
>
>
> > +                     break;
> >       } while (time_before(jiffies, timeout));
> >
> >       idpf_queue_assign(GEN_CHK, complq, gen_flag);
> > --
> > 2.52.0.351.gbe84eed79e-goog