RE: [PATCH net] net: xilinx: axienet: Increment Rx skb ring head pointer after BD is successfully allocated in dmaengine flow

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Hi Jakub,

-----Original Message-----
From: Jakub Kicinski <kuba@kernel.org>
Sent: Saturday, August 9, 2025 12:36 AM
To: Gupta, Suraj <redacted>
Cc: andrew+netdev@lunn.ch; davem@davemloft.net;
edumazet@google.com; pabeni@redhat.com; Simek, Michal
[off-list ref]; sean.anderson@linux.dev; Pandey, Radhey
Shyam [off-list ref]; horms@kernel.org;
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Subject: Re: [PATCH net] net: xilinx: axienet: Increment Rx skb ring head
pointer after BD is successfully allocated in dmaengine flow

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On Wed, 6 Aug 2025 00:49:58 +0530 Suraj Gupta wrote:

quoted

In DMAengine flow, AXI DMA driver invokes callback before freeing BD
in irq handling path.
In Rx callback (axienet_dma_rx_cb()), axienet driver tries to allocate
new BD after processing skb.
This will be problematic if both AXI-DMA and AXI ethernet have same BD
count as all Rx BDs will be allocated initially and it won't be able
to allocate new one after Rx irq. Incrementing head pointer w/o
checking for BD allocation will result in garbage values in skb BD and
cause the below kernel crash:

Unable to handle kernel paging request at virtual address
fffffffffffffffa <snip> Internal error: Oops: 0000000096000006 [#1]
SMP pc : axienet_dma_rx_cb+0x78/0x150 lr :
axienet_dma_rx_cb+0x78/0x150  Call trace:
  axienet_dma_rx_cb+0x78/0x150 (P)
  xilinx_dma_do_tasklet+0xdc/0x290
  tasklet_action_common+0x12c/0x178
  tasklet_action+0x30/0x3c
  handle_softirqs+0xf8/0x230
<snip>

Do you mean that we're incrementing lp->rx_ring_head before we know that
the submission will succeed? Potentially leaving an uninitialized entry (say at
index n), next attempt will try to use the next entry (n + 1) but the completion
will not know about the skip so it will try to complete entry n ?

This is really not coming thru in your explanation.

You're right, I only explained the issue I faced while running perf test with same BD count in axienet and AXI DMA. Above is more generic explanation of the situation here. I'll modify the description.

The fix itself seems incomplete. Even if we correctly skip the increment we
will never try to catch up with the allocations, the ring will have fewer
outstanding Rx skbs until reset, right? Worst case we drop all the skbs and the
ring will be empty, no Rx will happen until reset.
The shutdown path seems to be checking for skb = NULL so I guess it's correct
but good to double check..
--
pw-bot: cr

I agree that Rx ring will have fewer outstanding skbs. But I think that difference won't exceed one anytime as descriptors submission will fail only once due to insufficient space in AXIDMA BD ring. Rest of the time we already will have an extra entry in AXIDMA BD ring. Also, invoking callback (where Rx skb ring hp is filled in axienet)and freeing AXIDMA BD are part of same tasklet in AXIDMA driver so next callback will only be called after freeing a BD.
I tested running stress tests (Both UPD and TCP netperf).
Please let me know your thoughts if I'm missing something.

Thanks,
Suraj