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

[Sean Anderson <sean.anderson@linux.dev>]

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@linux.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