netdev - Re: [PATCH v2] net/mlx5e: Use refcount

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Date:   Mon, 5 Aug 2019 14:55:34 +0800
From:   Chuhong Yuan <hslester96@...il.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        "David S . Miller" <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>, linux-rdma@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] net/mlx5e: Use refcount_t for refcount

On Mon, Aug 5, 2019 at 2:13 PM Leon Romanovsky <leon@...nel.org> wrote:
>
> On Sun, Aug 04, 2019 at 10:44:47PM +0800, Chuhong Yuan wrote:
> > On Sun, Aug 4, 2019 at 8:59 PM Leon Romanovsky <leon@...nel.org> wrote:
> > >
> > > On Sat, Aug 03, 2019 at 12:48:28AM +0800, Chuhong Yuan wrote:
> > > > refcount_t is better for reference counters since its
> > > > implementation can prevent overflows.
> > > > So convert atomic_t ref counters to refcount_t.
> > >
> > > I'm not thrilled to see those automatic conversion patches, especially
> > > for flows which can't overflow. There is nothing wrong in using atomic_t
> > > type of variable, do you have in mind flow which will cause to overflow?
> > >
> > > Thanks
> >
> > I have to say that these patches are not done automatically...
> > Only the detection of problems is done by a script.
> > All conversions are done manually.
>
> Even worse, you need to audit usage of atomic_t and replace there
> it can overflow.
>
> >
> > I am not sure whether the flow can cause an overflow.
>
> It can't.
>
> > But I think it is hard to ensure that a data path is impossible
> > to have problems in any cases including being attacked.
>
> It is not data path, and I doubt that such conversion will be allowed
> in data paths without proving that no performance regression is introduced.
>>
>
> > So I think it is better to do this minor revision to prevent
> > potential risk, just like we have done in mlx5/core/cq.c.
>
> mlx5/core/cq.c is a different beast, refcount there means actual users
> of CQ which are limited in SW, so in theory, they have potential
> to be overflown.
>
> It is not the case here, there your are adding new port.
> There is nothing wrong with atomic_t.
>

Thanks for your explanation!
I will pay attention to this point in similar cases.
But it seems that the semantic of refcount is not always as clear as here...

Regards,
Chuhong


> Thanks
>
> >
> > Regards,
> > Chuhong
> >
> > > >
> > > > Signed-off-by: Chuhong Yuan <hslester96@...il.com>
> > > > ---
> > > > Changes in v2:
> > > >   - Add #include.
> > > >
> > > >  drivers/net/ethernet/mellanox/mlx5/core/lib/vxlan.c | 9 +++++----
> > > >  1 file changed, 5 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/vxlan.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/vxlan.c
> > > > index b9d4f4e19ff9..148b55c3db7a 100644
> > > > --- a/drivers/net/ethernet/mellanox/mlx5/core/lib/vxlan.c
> > > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/vxlan.c
> > > > @@ -32,6 +32,7 @@
> > > >
> > > >  #include <linux/kernel.h>
> > > >  #include <linux/module.h>
> > > > +#include <linux/refcount.h>
> > > >  #include <linux/mlx5/driver.h>
> > > >  #include <net/vxlan.h>
> > > >  #include "mlx5_core.h"
> > > > @@ -48,7 +49,7 @@ struct mlx5_vxlan {
> > > >
> > > >  struct mlx5_vxlan_port {
> > > >       struct hlist_node hlist;
> > > > -     atomic_t refcount;
> > > > +     refcount_t refcount;
> > > >       u16 udp_port;
> > > >  };
> > > >
> > > > @@ -113,7 +114,7 @@ int mlx5_vxlan_add_port(struct mlx5_vxlan *vxlan, u16 port)
> > > >
> > > >       vxlanp = mlx5_vxlan_lookup_port(vxlan, port);
> > > >       if (vxlanp) {
> > > > -             atomic_inc(&vxlanp->refcount);
> > > > +             refcount_inc(&vxlanp->refcount);
> > > >               return 0;
> > > >       }
> > > >
> > > > @@ -137,7 +138,7 @@ int mlx5_vxlan_add_port(struct mlx5_vxlan *vxlan, u16 port)
> > > >       }
> > > >
> > > >       vxlanp->udp_port = port;
> > > > -     atomic_set(&vxlanp->refcount, 1);
> > > > +     refcount_set(&vxlanp->refcount, 1);
> > > >
> > > >       spin_lock_bh(&vxlan->lock);
> > > >       hash_add(vxlan->htable, &vxlanp->hlist, port);
> > > > @@ -170,7 +171,7 @@ int mlx5_vxlan_del_port(struct mlx5_vxlan *vxlan, u16 port)
> > > >               goto out_unlock;
> > > >       }
> > > >
> > > > -     if (atomic_dec_and_test(&vxlanp->refcount)) {
> > > > +     if (refcount_dec_and_test(&vxlanp->refcount)) {
> > > >               hash_del(&vxlanp->hlist);
> > > >               remove = true;
> > > >       }
> > > > --
> > > > 2.20.1
> > > >