netdev - Re: [PATCH v17 10/22] cx/memdev: Indicate probe deferral

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Message-ID: <b29bf20f-456f-4772-959b-2287ec0f54d4@intel.com>
Date: Tue, 1 Jul 2025 09:25:44 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: Alejandro Lucero Palau <alucerop@....com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: alejandro.lucero-palau@....com, linux-cxl@...r.kernel.org,
 netdev@...r.kernel.org, dan.j.williams@...el.com, edward.cree@....com,
 davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, edumazet@...gle.com
Subject: Re: [PATCH v17 10/22] cx/memdev: Indicate probe deferral



On 7/1/25 9:07 AM, Alejandro Lucero Palau wrote:
> 
> On 6/30/25 17:20, Jonathan Cameron wrote:
>> Hi Dave,
>>
>>>> +/*
>>>> + * Try to get a locked reference on a memdev's CXL port topology
>>>> + * connection. Be careful to observe when cxl_mem_probe() has deposited
>>>> + * a probe deferral awaiting the arrival of the CXL root driver.
>>>> + */
>>>> +struct cxl_port *cxl_acquire_endpoint(struct cxl_memdev *cxlmd)
>> Just focusing on this part.
>>
>>> Annotation of __acquires() is needed here to annotate that this function is taking multiple locks and keeping the locks.
>> Messy because it's a conditional case and on error we never have
>> a call marked __releases() so sparse may moan.
>>
>> In theory we have __cond_acquires() but I think the sparse tooling
>> is still missing for that.
>>
>> One option is to hike the thing into a header as inline and use __acquire()
>> in the appropriate places.  Then sparse can see the markings
>> without problems.
>>
>> https://lore.kernel.org/all/20250305161652.GA18280@noisy.programming.kicks-ass.net/
>>
>> has some discussion on fixing the annotation issues around conditional locks
>> for LLVM but for now I think we are still stuck.
>>
>> For the original __cond_acquires()
>> https://lore.kernel.org/all/CAHk-=wjZfO9hGqJ2_hGQG3U_XzSh9_XaXze=HgPdvJbgrvASfA@mail.gmail.com/
>>
>> Linus posted sparse and kernel support but I think only the kernel bit merged
>> as sparse is currently (I think) unmaintained.
>>
> 
> Not sure what is the conclusion to this: should I do it or not?

Sounds like we can't with the way it's conditionally done.
> 
> 
> I can not see the __acquires being used yet by cxl core so I wonder if this needs to be introduced only when new code is added or it should require a core revision for adding all required. I mean, those locks being used in other code parts but not "advertised" by __acquires, is not that a problem?

It's only needed if you acquire a lock and leaving it held and then releases it in a different function. That allows sparse(?) to track if you are locking correctly. You don't need it if it's being done in the same function.

DJ


> 
> 
>>>> +{
>>>> +    struct cxl_port *endpoint;
>>>> +    int rc = -ENXIO;
>>>> +
>>>> +    device_lock(&cxlmd->dev);
>>>> +> +    endpoint = cxlmd->endpoint;
>>>> +    if (!endpoint)
>>>> +        goto err;
>>>> +
>>>> +    if (IS_ERR(endpoint)) {
>>>> +        rc = PTR_ERR(endpoint);
>>>> +        goto err;
>>>> +    }
>>>> +
>>>> +    device_lock(&endpoint->dev);
>>>> +    if (!endpoint->dev.driver)> +        goto err_endpoint;
>>>> +
>>>> +    return endpoint;
>>>> +
>>>> +err_endpoint:
>>>> +    device_unlock(&endpoint->dev);
>>>> +err:
>>>> +    device_unlock(&cxlmd->dev);
>>>> +    return ERR_PTR(rc);
>>>> +}
>>>> +EXPORT_SYMBOL_NS_GPL(cxl_acquire_endpoint, "CXL");
>>>> +
>>>> +void cxl_release_endpoint(struct cxl_memdev *cxlmd, struct cxl_port *endpoint)
>>> And __releases() here to release the lock annotations
>>>> +{
>>>> +    device_unlock(&endpoint->dev);
>>>> +    device_unlock(&cxlmd->dev);
>>>> +}
>>>> +EXPORT_SYMBOL_NS_GPL(cxl_release_endpoint, "CXL");
>>