Re: [PATCH v6 11/22] ACPI: platform_profile: Add name attribute to class interface

[Mario Limonciello <mario.limonciello@amd.com>]

On 11/19/2024 06:26, Armin Wolf wrote:
> Am 19.11.24 um 05:09 schrieb Mario Limonciello:
> 
>> On 11/18/2024 18:28, Armin Wolf wrote:
>>> Am 18.11.24 um 20:43 schrieb Armin Wolf:
>>>
>>>> Am 09.11.24 um 05:41 schrieb Mario Limonciello:
>>>>
>>>>> The name attribute shows the name of the associated platform profile
>>>>> handler.
>>>>>
>>>>> Tested-by: Mark Pearson <mpearson-lenovo@squebb.ca>
>>>>> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
>>>>> ---
>>>>>   drivers/acpi/platform_profile.c | 27 +++++++++++++++++++++++++++
>>>>>   1 file changed, 27 insertions(+)
>>>>>
>>>>> diff --git a/drivers/acpi/platform_profile.c
>>>>> b/drivers/acpi/platform_profile.c
>>>>> index ef6af2c655524..4e2eda18f7f5f 100644
>>>>> --- a/drivers/acpi/platform_profile.c
>>>>> +++ b/drivers/acpi/platform_profile.c
>>>>> @@ -25,8 +25,35 @@ static_assert(ARRAY_SIZE(profile_names) ==
>>>>> PLATFORM_PROFILE_LAST);
>>>>>
>>>>>   static DEFINE_IDA(platform_profile_ida);
>>>>>
>>>>> +/**
>>>>> + * name_show - Show the name of the profile handler
>>>>> + * @dev: The device
>>>>> + * @attr: The attribute
>>>>> + * @buf: The buffer to write to
>>>>> + * Return: The number of bytes written
>>>>> + */
>>>>> +static ssize_t name_show(struct device *dev,
>>>>> +             struct device_attribute *attr,
>>>>> +             char *buf)
>>>>> +{
>>>>> +    struct platform_profile_handler *handler = dev_get_drvdata(dev);
>>>>> +
>>>>> +    scoped_cond_guard(mutex_intr, return -ERESTARTSYS,
>>>>> &profile_lock) {
>>>>> +        return sysfs_emit(buf, "%s\n", handler->name);
>>>>> +    }
>>>>> +    return -ERESTARTSYS;
>>>>
>>>> I still have a bad feeling about the locking inside the class
>>>> attributes...
>>>>
>>>> Can we assume that no sysfs accesses occur after unregistering the
>>>> class device?
>>>>
>>>> Even if this is not the case then the locking fails to protect the
>>>> platform_profile_handler here.
>>>> If the device is unregistered right after dev_get_drvdata() was
>>>> called, then we would sill operate
>>>> on possibly stale data once we take the profile_lock.
>>>>
>>>> Does someone have any clue how sysfs attributes act during removal?
>>>>
>>> I think i found the answer to my questions inside this patch series:
>>> https://lore.kernel.org/linux-kernel/1390951311-15325-1-git-send-email-
>>> tj@kernel.org
>>>
>>> It says that:
>>>
>>>      kernfs / sysfs implement the "sever" semantic for userland
>>> accesses.
>>>      When a node is removed, no further userland operations are
>>> allowed and
>>>      the in-flight ones are drained before removal is finished. This
>>> makes
>>>      policing post-mortem userland accesses trivial for its users.
>>>
>>> In this case taking the profile_lock when reading/writing class
>>> attributes seems to be unnecessary.
>>> Please remove the unnecessary locking inside the class attributes.
>>>
>>
>> Before I respin a v7, let's make sure we're agreed on which things
>> need locking and which don't.
>>
>> Functions that check if a lock is held:
>> _store_class_profile()
>> _notify_class_profile()
>> get_class_profile()
>> _aggregate_choices()
>>
>> Functions that take a lock:
>> name_show()
>> choices_show()
>> profile_show()
>> profile_store()
>> platform_profile_choices_show()
>> platform_profile_show()
>> platform_profile_store()
>> platform_profile_cycle()
>> platform_profile_register()
>> platform_profile_remove()
>>
>> Functions that don't take or check for a lock (these are intermediary
>> and things they call check for one):
>> _aggregate_profiles()
>> _store_and_notify()
>>
>> Are you suggesting that basically these 4 can drop taking the lock?
>> name_show()
>> choices_show()
>> profile_show()
>> profile_store()
>>
>> I think the show() ones I can get behind, but I'm worried about
>> profile_store(), particularly as it pertains to the other callers of
>> _store_class_profile() because it's incongruent how the other callers
>> would use it then.
>>
>> Can we perhaps just drop it for the 3 class attribute show() ones?
> 
> I think so, i also remembered that profile_store() needs to keep taking 
> the lock in case platform_profile_cycle() is currently
> running.

Actually considering this, we need to keep it on profile_show() too then 
for the exact same reason.

I will drop it for choices and name though.

> 
> Can you also remove the second call to dev_get_drvdata() in 
> _store_class_profile()?
> 

Sure.

> Thanks,
> Armin Wolf
> 
>>
>> LMK.
>>
>>
>>