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Date: Tue, 30 Jan 2024 08:35:39 +0100
From: Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To: Jisheng Zhang <jszhang@kernel.org>
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Subject: Re: [PATCH net v2] net: stmmac: protect updates of 64-bit
 statistics counters
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On Tue, 30 Jan 2024 13:00:10 +0800
Jisheng Zhang <jszhang@kernel.org> wrote:

> On Sun, Jan 28, 2024 at 08:35:29PM +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@torres.zugschlus.de/t/
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Petr Tesarik <petr@tesarici.cz>  
> 
> Thanks for the fix patch. One trivial improviement below
> s/netdev_alloc_pcpu_stats/devm_netdev_alloc_pcpu_stats to simplify
> error and exit code path.

Thanks for your review.

In fact, many other allocations in stmmac could be converted to devm_*.
I wanted to stay consistent with the existing code, but hey, you're
right there's no good reason for it.

Plus, I can send convert the other places with another patch.

> With that:
> Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
> 
> PS: when I sent the above "net: stmmac: use per-queue 64 bit statistics
> where necessary", I had an impression: there are too much statistics in
> stmmac driver, I didn't see so many statistics in other eth drivers, is
> it possible to remove some useless or not that useful statistic members?

I don't feel authorized to make the decision. But I also wonder about
some counters. For example, why is there tx_packets and tx_pkt_n? The
former is shown as RX packets by "ip stats show dev end0", the latter
is shown by as tx_pkt_n by "ethtools -S end0". The values do differ,
but I have no clue why, and if they are even expected to be different
or if it's a bug.

Petr T