netdev - Re: [RESEND2 PATCH net v4 2/2] soc: fsl: qbman: Use raw spinlock for cgr

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Message-ID: <6764b9c5-b61a-4f20-a41a-125d5015a3e6@linux.dev>
Date: Tue, 5 Mar 2024 13:14:09 -0500
From: Sean Anderson <sean.anderson@...ux.dev>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc: Vladimir Oltean <vladimir.oltean@....com>, Roy Pledge
 <roy.pledge@....com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "stable@...r.kernel.org" <stable@...r.kernel.org>,
 Li Yang <leoyang.li@....com>, Scott Wood <oss@...error.net>,
 Claudiu Manoil <claudiu.manoil@....com>,
 Camelia Groza <camelia.groza@....com>,
 Steffen Trumtrar <s.trumtrar@...gutronix.de>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
 "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RESEND2 PATCH net v4 2/2] soc: fsl: qbman: Use raw spinlock for
 cgr_lock

Hi,

On 2/23/24 11:02, Sean Anderson wrote:
> On 2/23/24 00:38, Christophe Leroy wrote:
>> Le 22/02/2024 à 18:07, Sean Anderson a écrit :
>>> [Vous ne recevez pas souvent de courriers de sean.anderson@...ux.dev. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
>>> 
>>> cgr_lock may be locked with interrupts already disabled by
>>> smp_call_function_single. As such, we must use a raw spinlock to avoid
>>> problems on PREEMPT_RT kernels. Although this bug has existed for a
>>> while, it was not apparent until commit ef2a8d5478b9 ("net: dpaa: Adjust
>>> queue depth on rate change") which invokes smp_call_function_single via
>>> qman_update_cgr_safe every time a link goes up or down.
>> 
>> Why a raw spinlock to avoid problems on PREEMPT_RT, can you elaborate ?
> 
> smp_call_function always runs its callback in hard IRQ context, even on
> PREEMPT_RT, where spinlocks can sleep. So we need to use raw spinlocks
> to ensure we aren't waiting on a sleeping task. See the first bug report
> for more discussion.
> 
> In the longer term it would be better to switch to some other
> abstraction.

Does this make sense to you?

--Sean