linux-kernel - Re: [PATCH] net: stmmac: protect statistics updates with a spinlock

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Date: Fri, 5 Jan 2024 10:58:42 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Petr Tesarik <petr@...arici.cz>
Cc: Alexandre Torgue <alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Chen-Yu Tsai <wens@...e.org>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, 
	"open list:STMMAC ETHERNET DRIVER" <netdev@...r.kernel.org>, 
	"moderated list:ARM/STM32 ARCHITECTURE" <linux-stm32@...md-mailman.stormreply.com>, 
	"moderated list:ARM/STM32 ARCHITECTURE" <linux-arm-kernel@...ts.infradead.org>, 
	open list <linux-kernel@...r.kernel.org>, 
	"open list:ARM/Allwinner sunXi SoC support" <linux-sunxi@...ts.linux.dev>
Subject: Re: [PATCH] net: stmmac: protect statistics updates with a spinlock

On Fri, Jan 5, 2024 at 10:16 AM Petr Tesarik <petr@...arici.cz> wrote:
>
> Add a spinlock to fix race conditions while updating Tx/Rx statistics.
>
> 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 actually observed on 32-bit Arm after stmmac_xmit()
> on one core raced with stmmac_napi_poll_tx() on another core.
>
> Signed-off-by: Petr Tesarik <petr@...arici.cz>

This is going to add more costs to 64bit platforms ?

It seems to me that the same syncp can be used from two different
threads : hard irq and napi poller...

At this point, I do not see why you keep linux/u64_stats_sync.h if you
decide to go for a spinlock...

Alternative would use atomic64_t fields for the ones where there is no
mutual exclusion.

RX : napi poll is definitely safe (protected by an atomic bit)
TX : each TX queue is also safe (protected by an atomic exclusion for
non LLTX drivers)

This leaves the fields updated from hardware interrupt context ?