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

[Christophe Leroy]

Le 05/03/2024 à 19:14, Sean Anderson a écrit :
> [Vous ne recevez pas souvent de courriers de sean.anderson@xxxxxxxxx. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
> 
> 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@xxxxxxxxx. 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?

Yes that fine, thanks for the clarification. Maybe you can explain that 
in the patch description in case you send a v5.

Christophe