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Date:   Fri, 13 Dec 2019 17:10:08 +0000
From:   Maxim Mikityanskiy <maximmi@...lanox.com>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Björn Töpel <bjorn.topel@...il.com>
CC:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "David S. Miller" <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Maxim Mikityanskiy <maxtram95@...il.com>
Subject: Re: [PATCH bpf 3/4] net/i40e: Fix concurrency issues between config
 flow and XSK

On 2019-12-11 12:08, Björn Töpel wrote:
> On 2019-12-10 15:26, Maxim Mikityanskiy wrote:
>> On 2019-12-06 11:03, Björn Töpel wrote:
>>> On Thu, 5 Dec 2019 at 16:52, Maxim Mikityanskiy 
>>> <maximmi@...lanox.com> wrote:
>>>>
>>>> Use synchronize_rcu to wait until the XSK wakeup function finishes
>>>> before destroying the resources it uses:
>>>>
>>>> 1. i40e_down already calls synchronize_rcu. On i40e_down either
>>>> __I40E_VSI_DOWN or __I40E_CONFIG_BUSY is set. Check the latter in
>>>> i40e_xsk_async_xmit (the former is already checked there).
>>>>
>>>> 2. After switching the XDP program, call synchronize_rcu to let
>>>> i40e_xsk_async_xmit exit before the XDP program is freed.
>>>>
>>>> 3. Changing the number of channels brings the interface down (see
>>>> i40e_prep_for_reset and i40e_pf_quiesce_all_vsi).
>>>>
>>>> 4. Disabling UMEM sets __I40E_CONFIG_BUSY, too.
>>>>
>>>> Signed-off-by: Maxim Mikityanskiy <maximmi@...lanox.com>
>>>> ---
>>>>    drivers/net/ethernet/intel/i40e/i40e_main.c | 7 +++++--
>>>>    drivers/net/ethernet/intel/i40e/i40e_xsk.c  | 4 ++++
>>>>    2 files changed, 9 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c 
>>>> b/drivers/net/ethernet/intel/i40e/i40e_main.c
>>>> index 1ccabeafa44c..afa3a99e68e1 100644
>>>> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
>>>> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
>>>> @@ -6823,8 +6823,8 @@ void i40e_down(struct i40e_vsi *vsi)
>>>>           for (i = 0; i < vsi->num_queue_pairs; i++) {
>>>>                   i40e_clean_tx_ring(vsi->tx_rings[i]);
>>>>                   if (i40e_enabled_xdp_vsi(vsi)) {
>>>> -                       /* Make sure that in-progress ndo_xdp_xmit
>>>> -                        * calls are completed.
>>>> +                       /* Make sure that in-progress ndo_xdp_xmit and
>>>> +                        * ndo_xsk_async_xmit calls are completed.
>>
>> Ooops, I see now some changes were lost during rebasing. I had a version
>> of this series with ndo_xsk_async_xmit -> ndo_xsk_wakeup fixed.
>>
>>>>                            */
>>>>                           synchronize_rcu();
>>>>                           i40e_clean_tx_ring(vsi->xdp_rings[i]);
>>>> @@ -12545,6 +12545,9 @@ static int i40e_xdp_setup(struct i40e_vsi *vsi,
>>>>                   i40e_prep_for_reset(pf, true);
>>>>
>>>>           old_prog = xchg(&vsi->xdp_prog, prog);
>>>> +       if (old_prog)
>>>> +               /* Wait until ndo_xsk_async_xmit completes. */
>>>> +               synchronize_rcu();
>>>
>>> This is not needed -- please correct me if I'm missing something! If
>>> we're disabling XDP, the need_reset-clause will take care or the
>>> proper synchronization.
>>
>> Yes, it was added to cover the case of disabling XDP. I'm not sure how
>> i40e_reset_and_rebuild will help here. What I wanted to achieve is to
>> wait until all i40e_xsk_wakeups finish after setting vsi->xdp_prog =
>> NULL and before doing anything else. I don't see how
>> i40e_reset_and_rebuild helps here, but I'm not very familiar with i40e
>> code. Could you guide me through the code of i40e_reset_and_rebuild and
>> show how it takes care of the synchronization?
>>
>>> And we don't need to worry about old_prog for
>>> the swap-XDP case,
>>
>> Right.
>>
>>> because bpf_prog_put() does cleanup with
>>> call_rcu().
>>
>> But if i40e_xsk_wakeup is not wrapped with rcu_read_lock, it won't sync
>> with it.
>>
>>>
>>>>
>>>>           if (need_reset)
>>>>                   i40e_reset_and_rebuild(pf, true, true);
>>>> diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c 
>>>> b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
>>>> index d07e1a890428..f73cd917c44f 100644
>>>> --- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
>>>> +++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
>>>> @@ -787,8 +787,12 @@ int i40e_xsk_wakeup(struct net_device *dev, u32 
>>>> queue_id, u32 flags)
>>>>    {
>>>>           struct i40e_netdev_priv *np = netdev_priv(dev);
>>>>           struct i40e_vsi *vsi = np->vsi;
>>>> +       struct i40e_pf *pf = vsi->back;
>>>>           struct i40e_ring *ring;
>>>>
>>>> +       if (test_bit(__I40E_CONFIG_BUSY, pf->state))
>>>> +               return -ENETDOWN;
>>>> +
>>>
>>> You right that we need to check for BUSY, since the XDP ring might be
>>> stale! Thanks for catching this! Can you respin this patch, with just
>>> this hunk? (Unless I'm wrong! :-))
>>
>> This check itself will not be enough, unless you wrap i40e_xsk_wakeup
>> with rcu_read_lock.
>>
>> i40e_xsk_wakeup does some checks in the beginning, then the main part of
>> the code goes. My assumption is that if the checks don't pass, the main
>> part will fail in some way (e.g., NULL or dangling pointer when
>> accessing the ring), otherwise you wouldn't need those checks at all.
>> Without RCU, even if you do these checks, it may still fail in the
>> following scenario:
>>
>> 1. i40e_xsk_wakeup starts running, checks the flag, the flag is set, the
>> function goes on.
>>
>> 2. The flag gets cleared.
>>
>> 3. The resources are destroyed.
>>
>> 4. i40e_xsk_wakeup tries to access the resources that were destroyed.
>>
>> I don't see anything in i40e and ixgbe that currently protects from such
>> a race condition. If I'm missing anything, please point me to it,
>> otherwise i40e_xsk_wakeup really needs to be wrapped into rcu_read_lock.
>> I would prefer to have rcu_read_lock in the kernel, so that all drivers
>> could benefit from it, because I think it's the same issue in all
>> drivers, but I'm fine with moving it to the drivers if there is a reason
>> why some drivers may not need it.
>>
>> Thanks for taking a look. Let's settle the case with Intel's drivers, so
>> that I will know what fixes to include into the respin.
>>
> 
> Max, you're right. It's not correct without RCU locking. Thanks for the
> patience.
> 
> For the Intel ndo_xsk_wakeup implementation we need to make sure that 
> the 1. umem(socket) exists, and that the 2. XDP rings exists for the 
> duration of the call.
> 
> 1. In xsk_unbind_dev() the state is changed to UNBOUND and then
>     followed by synchronize_net(). It would be, as you wrote, incorrect
>     without RCU locking the ndo_xsk_wakeup() region.
> 
> 2. To ensure that the XDP rings are intact for the duration of the
>     ndo_xsk_wakeup(), a synchronize_rcu() is required when the XDP
>     program is swapped out (prog->NULL).
> 
> Again, both 1 and 2 requires RCU locking.
> 
> What do you think about the this patch for xsk.c (and the Intel 
> drivers)? It moves all ndo_xsk_wakeup calls do one place.

Looks good, please also see a few comments below.

Checking the __I40E_CONFIG_BUSY and __IXGBE_TX_DISABLED flags in the 
wakeup functions is still required, I believe. You didn't include these 
parts into your patch, I just want to make sure you don't suggest to 
drop these parts of my patches.

How would you like to proceed with it? Do you want me to integrate your 
changes into my patches and respin with both my and your sign-offs?

> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e.h 
> b/drivers/net/ethernet/intel/i40e/i40e.h
> index cb6367334ca7..4833187bd259 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e.h
> +++ b/drivers/net/ethernet/intel/i40e/i40e.h
> @@ -1152,7 +1152,7 @@ void i40e_set_fec_in_flags(u8 fec_cfg, u32 *flags);
> 
>   static inline bool i40e_enabled_xdp_vsi(struct i40e_vsi *vsi)
>   {
> -    return !!vsi->xdp_prog;
> +    return !!READ_ONCE(vsi->xdp_prog);
>   }
> 
>   int i40e_create_queue_channel(struct i40e_vsi *vsi, struct 
> i40e_channel *ch);
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c 
> b/drivers/net/ethernet/intel/i40e/i40e_main.c
> index 1ccabeafa44c..fd084f03f00d 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> @@ -12546,8 +12546,11 @@ static int i40e_xdp_setup(struct i40e_vsi *vsi,
> 
>       old_prog = xchg(&vsi->xdp_prog, prog);
> 
> -    if (need_reset)
> +    if (need_reset) {
> +        if (!prog)
> +            synchronize_rcu();
>           i40e_reset_and_rebuild(pf, true, true);
> +    }
> 
>       for (i = 0; i < vsi->num_queue_pairs; i++)
>           WRITE_ONCE(vsi->rx_rings[i]->xdp_prog, vsi->xdp_prog);
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c 
> b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index 25c097cd8100..adff2cbcde1a 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -10261,7 +10261,11 @@ static int ixgbe_xdp_setup(struct net_device 
> *dev, struct bpf_prog *prog)
> 
>       /* If transitioning XDP modes reconfigure rings */
>       if (need_reset) {
> -        int err = ixgbe_setup_tc(dev, adapter->hw_tcs);
> +        int err;
> +
> +        if (!prog)
> +            synchronize_rcu();
> +        err = ixgbe_setup_tc(dev, adapter->hw_tcs);
> 
>           if (err) {
>               rcu_assign_pointer(adapter->xdp_prog, old_prog);
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 956793893c9d..dbf06c3b7061 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -334,12 +334,20 @@ bool xsk_umem_consume_tx(struct xdp_umem *umem, 
> struct xdp_desc *desc)
>   }
>   EXPORT_SYMBOL(xsk_umem_consume_tx);
> 
> -static int xsk_zc_xmit(struct xdp_sock *xs)
> +static int xsk_wakeup(struct xdp_sock *xs, u8 flags)
>   {
>       struct net_device *dev = xs->dev;
> +    int err;
> 
> -    return dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id,
> -                           XDP_WAKEUP_TX);
> +    rcu_read_lock();
> +    err = dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id, flags);
> +    rcu_read_unlock();
> +    return err;
> +}
> +
> +static int xsk_zc_xmit(struct xdp_sock *xs)
> +{
> +    return xsk_wakeup(xs, XDP_WAKEUP_TX);
>   }
> 
>   static void xsk_destruct_skb(struct sk_buff *skb)
> @@ -453,19 +461,16 @@ static __poll_t xsk_poll(struct file *file, struct 
> socket *sock,
>       __poll_t mask = datagram_poll(file, sock, wait);
>       struct sock *sk = sock->sk;
>       struct xdp_sock *xs = xdp_sk(sk);
> -    struct net_device *dev;
>       struct xdp_umem *umem;
> 
>       if (unlikely(!xsk_is_bound(xs)))
>           return mask;
> 
> -    dev = xs->dev;
>       umem = xs->umem;
> 
>       if (umem->need_wakeup) {
> -        if (dev->netdev_ops->ndo_xsk_wakeup)
> -            dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id,
> -                            umem->need_wakeup);
> +        if (xs->zc)
> +            xsk_wakeup(umem->need_wakeup);

Thanks for spotting this. I missed the second place where wakeup is called.

BTW, does the current code contain a bug? The socket can be opened in 
copy mode even if ndo_xsk_wakeup is not NULL, and we'll call 
ndo_xsk_wakeup in this case, although it's not zero-copy. After your 
patch it will be fixed.

P.S. I'll be on vacation, so I added my personal email to Cc to be able 
to receive responses.

>           else
>               /* Poll needs to drive Tx also in copy mode */
>               __xsk_sendmsg(sk);
> 

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