linux-kernel - Re: [Intel-wired-lan] [PATCH] [PATCH iwl-net] idpf: Acquire the lock before accessing the xn->salt

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Message-ID: <CA+M8utN7FbwMF5QN8O0a0Qnd3ykQwq7O4QkHMVEaBj2jE9BEYw@mail.gmail.com>
Date: Wed, 7 Aug 2024 06:58:59 -0700
From: Manoj Vishwanathan <manojvishy@...gle.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: netdev@...r.kernel.org, David Decotigny <decot@...gle.com>, linux-kernel@...r.kernel.org, 
	Tony Nguyen <anthony.l.nguyen@...el.com>, Przemek Kitszel <przemyslaw.kitszel@...el.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	intel-wired-lan@...ts.osuosl.org
Subject: Re: [Intel-wired-lan] [PATCH] [PATCH iwl-net] idpf: Acquire the lock
 before accessing the xn->salt

Thanks Przemek & Olek for your quick feedback and responses.
Hi Olek,
I can add more details about the issue we faced in the commit message.
The bug we had here was a virtchnl delay leading to the xn->salt
mismatch. This could be due to several factors including default CPU
bounded kworker workqueue for virtchnl message processing being
starved by aggressive userspace load causing the virtchnl to be
delayed. While debugging this issue, this locking order  appeared like
a potential issue, hence the change was made.
But, this change is more a clean up we felt based on concurrent access
to the virtchnl transaction struct and does not fix the issue. This is
more of the patch to do the right thing before we access the "xn".
I wanted to start with a first patch to the community for acceptance
followed by a series of other patches that are general clean up or
improvements to IDPF in general. Will follow with with [PATCH v3]

Thanks,
Manoj

On Wed, Aug 7, 2024 at 4:05 AM Alexander Lobakin
<aleksander.lobakin@...el.com> wrote:
>
> From: Manoj Vishwanathan <manojvishy@...gle.com>
> Date: Mon,  5 Aug 2024 18:21:59 +0000
>
> > The transaction salt was being accessed before acquiring the
> > idpf_vc_xn_lock when idpf has to forward the virtchnl reply.
>
> You need to explain in details here what issue you have faced due to
> that, how to reproduce it and how this fix does help.
> Otherwise, it's impossible to suggest what is happening and how to test
> whether the fix is correct.
>
> >
> > Fixes: 34c21fa894a1a (“idpf: implement virtchnl transaction manager”)
> > Signed-off-by: Manoj Vishwanathan <manojvishy@...gle.com>
> > ---
> >  drivers/net/ethernet/intel/idpf/idpf_virtchnl.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/intel/idpf/idpf_virtchnl.c b/drivers/net/ethernet/intel/idpf/idpf_virtchnl.c
> > index 70986e12da28..30eec674d594 100644
> > --- a/drivers/net/ethernet/intel/idpf/idpf_virtchnl.c
> > +++ b/drivers/net/ethernet/intel/idpf/idpf_virtchnl.c
> > @@ -612,14 +612,15 @@ idpf_vc_xn_forward_reply(struct idpf_adapter *adapter,
> >               return -EINVAL;
> >       }
> >       xn = &adapter->vcxn_mngr->ring[xn_idx];
> > +     idpf_vc_xn_lock(xn);
> >       salt = FIELD_GET(IDPF_VC_XN_SALT_M, msg_info);
>
> The lock can be taken here after the FIELD_GET(), not before, to reduce
> the critical/locked section execution time.
>
> >       if (xn->salt != salt) {
> >               dev_err_ratelimited(&adapter->pdev->dev, "Transaction salt does not match (%02x != %02x)\n",
> >                                   xn->salt, salt);
> > +             idpf_vc_xn_unlock(xn);
> >               return -EINVAL;
> >       }
> >
> > -     idpf_vc_xn_lock(xn);
> >       switch (xn->state) {
> >       case IDPF_VC_XN_WAITING:
> >               /* success */
>
> Thanks,
> Olek