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Message-ID: <ddbae662-96d6-8779-eb8a-5a375e97ec22@molgen.mpg.de>
Date:   Sun, 26 Mar 2023 17:03:56 +0200
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Subject: Re: [Intel-wired-lan] [PATCH net v3] ixgbe: Panic during XDP_TX with
 > 64 CPUs
To:     John Hickey <jjh@daedalian.us>
Cc:     Tony Nguyen <anthony.l.nguyen@intel.com>,
        Shujin Li <lishujin@kuaishou.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, netdev@vger.kernel.org,
        John Fastabend <john.fastabend@gmail.com>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Eric Dumazet <edumazet@google.com>,
        intel-wired-lan@lists.osuosl.org, Jakub Kicinski <kuba@kernel.org>,
        bpf@vger.kernel.org, Paolo Abeni <pabeni@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        linux-kernel@vger.kernel.org, Jason Xing <xingwanli@kuaishou.com>
References: <20230308220756.587317-1-jjh@daedalian.us>
Content-Language: en-US
From:   Paul Menzel <pmenzel@molgen.mpg.de>
In-Reply-To: <20230308220756.587317-1-jjh@daedalian.us>
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Precedence: bulk

Dear John,


Thank you for your patch.

I’d recommend, to use a statement in the git commit message/summary by 
adding a verb (in imperative mood). Maybe:

Fix panic during XDP_TX with > 64 CPUs

Am 08.03.23 um 23:07 schrieb John Hickey:
> In commit 'ixgbe: let the xdpdrv work with more than 64 cpus'
> (4fe815850bdc), support was added to allow XDP programs to run on systems

I think it’s more common to write it like:

In commit 4fe815850bdc (ixgbe: let the xdpdrv work with more than 64 cpus) …

Even shorter

Commit 4fe815850bdc (ixgbe: let the xdpdrv work with more than 64 cpus) 
adds support to allow XDP programs …

> with more than 64 CPUs by locking the XDP TX rings and indexing them
> using cpu % 64 (IXGBE_MAX_XDP_QS).
> 
> Upon trying this out patch via the Intel 5.18.6 out of tree driver

Upon trying this patch out via …

> on a system with more than 64 cores, the kernel paniced with an
> array-index-out-of-bounds at the return in ixgbe_determine_xdp_ring in
> ixgbe.h, which means ixgbe_determine_xdp_q_idx was just returning the
> cpu instead of cpu % IXGBE_MAX_XDP_QS.  An example splat:

Please add, that you have UBSAN  enabled, or does it happen without?

> 
>   ==========================================================================
>   UBSAN: array-index-out-of-bounds in
>   /var/lib/dkms/ixgbe/5.18.6+focal-1/build/src/ixgbe.h:1147:26
>   index 65 is out of range for type 'ixgbe_ring *[64]'
>   ==========================================================================
>   BUG: kernel NULL pointer dereference, address: 0000000000000058
>   #PF: supervisor read access in kernel mode
>   #PF: error_code(0x0000) - not-present page
>   PGD 0 P4D 0
>   Oops: 0000 [#1] SMP NOPTI
>   CPU: 65 PID: 408 Comm: ksoftirqd/65
>   Tainted: G          IOE     5.15.0-48-generic #54~20.04.1-Ubuntu
>   Hardware name: Dell Inc. PowerEdge R640/0W23H8, BIOS 2.5.4 01/13/2020
>   RIP: 0010:ixgbe_xmit_xdp_ring+0x1b/0x1c0 [ixgbe]
>   Code: 3b 52 d4 cf e9 42 f2 ff ff 66 0f 1f 44 00 00 0f 1f 44 00 00 55 b9
>   00 00 00 00 48 89 e5 41 57 41 56 41 55 41 54 53 48 83 ec 08 <44> 0f b7
>   47 58 0f b7 47 5a 0f b7 57 54 44 0f b7 76 08 66 41 39 c0

If you do not it yet, `scripts/decode_stacktrace.sh` helps decoding 
these traces.

>   RSP: 0018:ffffbc3fcd88fcb0 EFLAGS: 00010282
>   RAX: ffff92a253260980 RBX: ffffbc3fe68b00a0 RCX: 0000000000000000
>   RDX: ffff928b5f659000 RSI: ffff928b5f659000 RDI: 0000000000000000
>   RBP: ffffbc3fcd88fce0 R08: ffff92b9dfc20580 R09: 0000000000000001
>   R10: 3d3d3d3d3d3d3d3d R11: 3d3d3d3d3d3d3d3d R12: 0000000000000000
>   R13: ffff928b2f0fa8c0 R14: ffff928b9be20050 R15: 000000000000003c
>   FS:  0000000000000000(0000) GS:ffff92b9dfc00000(0000)
>   knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000000000058 CR3: 000000011dd6a002 CR4: 00000000007706e0
>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   PKRU: 55555554
>   Call Trace:
>    <TASK>
>    ixgbe_poll+0x103e/0x1280 [ixgbe]
>    ? sched_clock_cpu+0x12/0xe0
>    __napi_poll+0x30/0x160
>    net_rx_action+0x11c/0x270
>    __do_softirq+0xda/0x2ee
>    run_ksoftirqd+0x2f/0x50
>    smpboot_thread_fn+0xb7/0x150
>    ? sort_range+0x30/0x30
>    kthread+0x127/0x150
>    ? set_kthread_struct+0x50/0x50
>    ret_from_fork+0x1f/0x30
>    </TASK>
> 
> I think this is how it happens:
> 
> Upon loading the first XDP program on a system with more than 64 CPUs,
> ixgbe_xdp_locking_key is incremented in ixgbe_xdp_setup.  However,
> immediately after this, the rings are reconfigured by ixgbe_setup_tc.
> ixgbe_setup_tc calls ixgbe_clear_interrupt_scheme which calls
> ixgbe_free_q_vectors which calls ixgbe_free_q_vector in a loop.
> ixgbe_free_q_vector decrements ixgbe_xdp_locking_key once per call if
> it is non-zero.  Commenting out the decrement in ixgbe_free_q_vector
> stopped my system from panicing.
> 
> I suspect to make the original patch work, I would need to load an XDP
> program and then replace it in order to get ixgbe_xdp_locking_key back
> above 0 since ixgbe_setup_tc is only called when transitioning between
> XDP and non-XDP ring configurations, while ixgbe_xdp_locking_key is
> incremented every time ixgbe_xdp_setup is called.
> 
> Also, ixgbe_setup_tc can be called via ethtool --set-channels, so this
> becomes another path to decrement ixgbe_xdp_locking_key to 0 on systems
> with greater than 64 CPUs.

… with more than 64 CPUs.

> For this patch, I have changed static_branch_inc to static_branch_enable
> in ixgbe_setup_xdp.  We weren't counting references.  The
> ixgbe_xdp_locking_key only protects code in the XDP_TX path, which is
> not run when an XDP program is loaded.  The other condition for setting
> it on is the number of CPUs, which I assume is static.
> 
> Fixes: 4fe815850bdc ("ixgbe: let the xdpdrv work with more than 64 cpus")
> Signed-off-by: John Hickey <jjh@daedalian.us>
> ---
> v1 -> v2:
> 	Added Fixes and net tag.  No code changes.
> v2 -> v3:
> 	Added splat.  Slight clarification as to why ixgbe_xdp_locking_key
> 	is not turned off.  Based on feedback from Maciej Fijalkowski.
> ---
>   drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c  | 3 ---
>   drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 2 +-
>   2 files changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> index f8156fe4b1dc..0ee943db3dc9 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> @@ -1035,9 +1035,6 @@ static void ixgbe_free_q_vector(struct ixgbe_adapter *adapter, int v_idx)
>   	adapter->q_vector[v_idx] = NULL;
>   	__netif_napi_del(&q_vector->napi);
>   
> -	if (static_key_enabled(&ixgbe_xdp_locking_key))
> -		static_branch_dec(&ixgbe_xdp_locking_key);
> -
>   	/*
>   	 * after a call to __netif_napi_del() napi may still be used and
>   	 * ixgbe_get_stats64() might access the rings on this vector,
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index ab8370c413f3..cd2fb72c67be 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -10283,7 +10283,7 @@ static int ixgbe_xdp_setup(struct net_device *dev, struct bpf_prog *prog)
>   	if (nr_cpu_ids > IXGBE_MAX_XDP_QS * 2)
>   		return -ENOMEM;
>   	else if (nr_cpu_ids > IXGBE_MAX_XDP_QS)
> -		static_branch_inc(&ixgbe_xdp_locking_key);
> +		static_branch_enable(&ixgbe_xdp_locking_key);
>   
>   	old_prog = xchg(&adapter->xdp_prog, prog);
>   	need_reset = (!!prog != !!old_prog);


Kind regards,

Paul