Re: [PATCH net v3] net: stmmac: protect updates of 64-bit statistics counters

[Linux regression tracking (Thorsten Leemhuis)]

On 28.02.24 12:03, Petr Tesařík wrote:
> On Wed, 28 Feb 2024 07:19:56 +0100
> "Linux regression tracking (Thorsten Leemhuis)" <regressions@xxxxxxxxxxxxx> wrote:
> 
>> Net maintainers, chiming in here, as it seems handling this regression
>> stalled.
> Indeed, I was too busy with sandbox mode...

Hmm, no reply in the past week to Petr's request for help from someone
with more knowledge about the field. :-/

So I guess this means that this won't be fixed for 6.8? Unfortunate, but
well, that's how it it sometimes.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
Everything you wanna know about Linux kernel regression tracking:
https://linux-regtracking.leemhuis.info/about/#tldr
If I did something stupid, please tell me, as explained on that page.

#regzbot poke

>> On 13.02.24 16:52, Eric Dumazet wrote:
>>> On Tue, Feb 13, 2024 at 4:26 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:  
>>>> On Tue, Feb 13, 2024 at 03:51:35PM +0100, Eric Dumazet wrote:  
>>>>> On Tue, Feb 13, 2024 at 3:29 PM Jisheng Zhang <jszhang@xxxxxxxxxx> wrote:  
>>>>>> On Sun, Feb 11, 2024 at 08:30:21PM -0800, Guenter Roeck wrote:  
>>>>>>> On Sat, Feb 03, 2024 at 08:09:27PM +0100, Petr Tesarik wrote:  
>>>>>>>> As explained by a comment in <linux/u64_stats_sync.h>, write side of struct
>>>>>>>> u64_stats_sync must ensure mutual exclusion, or one seqcount update could
>>>>>>>> be lost on 32-bit platforms, thus blocking readers forever. Such lockups
>>>>>>>> have been observed in real world after stmmac_xmit() on one CPU raced with
>>>>>>>> stmmac_napi_poll_tx() on another CPU.
>>>>>>>>
>>>>>>>> To fix the issue without introducing a new lock, split the statics into
>>>>>>>> three parts:
>>>>>>>>
>>>>>>>> 1. fields updated only under the tx queue lock,
>>>>>>>> 2. fields updated only during NAPI poll,
>>>>>>>> 3. fields updated only from interrupt context,
>>>>>>>>
>>>>>>>> Updates to fields in the first two groups are already serialized through
>>>>>>>> other locks. It is sufficient to split the existing struct u64_stats_sync
>>>>>>>> so that each group has its own.
>>>>>>>>
>>>>>>>> Note that tx_set_ic_bit is updated from both contexts. Split this counter
>>>>>>>> so that each context gets its own, and calculate their sum to get the total
>>>>>>>> value in stmmac_get_ethtool_stats().
>>>>>>>>
>>>>>>>> For the third group, multiple interrupts may be processed by different CPUs
>>>>>>>> at the same time, but interrupts on the same CPU will not nest. Move fields
>>>>>>>> from this group to a newly created per-cpu struct stmmac_pcpu_stats.
>>>>>>>>
>>>>>>>> Fixes: 133466c3bbe1 ("net: stmmac: use per-queue 64 bit statistics where necessary")
>>>>>>>> Link: https://lore.kernel.org/netdev/Za173PhviYg-1qIn@xxxxxxxxxxxxxxxxxxx/t/
>>>>>>>> Cc: stable@xxxxxxxxxxxxxxx
>>>>>>>> Signed-off-by: Petr Tesarik <petr@xxxxxxxxxxx>  
>>>>>>>
>>>>>>> This patch results in a lockdep splat. Backtrace and bisect results attached.
>>>>>>>
>>>>>>> ---
>>>>>>> [   33.736728] ================================
>>>>>>> [   33.736805] WARNING: inconsistent lock state
>>>>>>> [   33.736953] 6.8.0-rc4 #1 Tainted: G                 N
>>>>>>> [   33.737080] --------------------------------
>>>>>>> [   33.737155] inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
>>>>>>> [   33.737309] kworker/0:2/39 [HC1[1]:SC0[2]:HE0:SE0] takes:
>>>>>>> [   33.737459] ef792074 (&syncp->seq#2){?...}-{0:0}, at: sun8i_dwmac_dma_interrupt+0x9c/0x28c
>>>>>>> [   33.738206] {HARDIRQ-ON-W} state was registered at:
>>>>>>> [   33.738318]   lock_acquire+0x11c/0x368
>>>>>>> [   33.738431]   __u64_stats_update_begin+0x104/0x1ac
>>>>>>> [   33.738525]   stmmac_xmit+0x4d0/0xc58  
>>>>>>
>>>>>> interesting lockdep splat...
>>>>>> stmmac_xmit() operates on txq_stats->q_syncp, while the
>>>>>> sun8i_dwmac_dma_interrupt() operates on pcpu's priv->xstats.pcpu_stats
>>>>>> they are different syncp. so how does lockdep splat happen.  
>>>>>
>>>>> Right, I do not see anything obvious yet.  
>>>>
>>>> Wild guess: I think it maybe saying that due to
>>>>
>>>>         inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
>>>>
>>>> the critical code may somehow be interrupted and, while handling the
>>>> interrupt, try to acquire the same lock again.  
>>>
>>> This should not happen, the 'syncp' are different. They have different
>>> lockdep classes.
>>>
>>> One is exclusively used from hard irq context.
>>>
>>> The second one only used from BH context.  
>>
>> Alexis Lothoré hit this now as well, see yesterday report in this
>> thread; apart from that nothing seem to have happened for two weeks now.
>> The change recently made it to some stable/longterm kernels, too. Makes
>> me wonder:
>>
>> What's the plan forward here? Is this considered to be a false positive?
> 
> Although my system has run stable for a couple of months, I am hesitant
> to call it a false positive.
> 
>> Or a real problem?
> 
> That's what I think. But I would have to learn a lot about the network
> stack to understand what exactly happens here.
> 
> It may go faster if somebody else on the Cc can give me a hint where to
> start looking based on the lockdep warning.
> 
> Petr T
> 
>