Re: [PATCH v1 01/11] seqlock: provide lockdep-free raw_seqcount_t variant

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On 17.12.21 18:02, Nadav Amit wrote:
> 
> 
>> On Dec 17, 2021, at 3:30 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> Sometimes it is required to have a seqcount implementation that uses
>> a structure with a fixed and minimal size -- just a bare unsigned int --
>> independent of the kernel configuration. This is especially valuable, when
>> the raw_ variants of the seqlock function will be used and the additional
>> lockdep part of the seqcount_t structure remains essentially unused.
>>
>> Let's provide a lockdep-free raw_seqcount_t variant that can be used via
>> the raw functions to have a basic seqlock.
>>
>> The target use case is embedding a raw_seqcount_t in the "struct page",
>> where we really want a minimal size and cannot tolerate a sudden grow of
>> the seqcount_t structure resulting in a significant "struct page"
>> increase or even a layout change.
>>
>> Provide raw_read_seqcount_retry(), to make it easy to match to
>> raw_read_seqcount_begin() in the code.
>>
>> Let's add a short documentation as well.
>>
>> Note: There might be other possible users for raw_seqcount_t where the
>>      lockdep part might be completely unused and just wastes memory --
>>      essentially any users that only use the raw_ function variants.
>>
> 
> Is it possible to force some policy when raw_seqcount_t is used to
> prevent its abuse? For instance not to allow to acquire other (certain?)
> locks when it is held?
> 

Good question ... in this series we won't be taking additional locks on
the reader or the writer side. Something like lockdep_forbid() /
lockdep_allow() to disallow any kind of locking. I haven't heard of
anything like that, maybe someone reading along has a clue?

The writer side might be easy to handle, but some seqcount operations
that don't do the full read()->retry() cycle are problematic
(->raw_read_seqcount).

> [ snip ]
> 
>> +/**
>> + * raw_seqcount_init() - runtime initializer for raw_seqcount_t
>> + * @s: Pointer to the raw_seqcount_t instance
>> + */
>> +# define raw_seqcount_init(s) __raw_seqcount_init(s)
>> +
>> #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>
>> # define SEQCOUNT_DEP_MAP_INIT(lockname)				\
>> @@ -111,11 +129,16 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
>> # define seqcount_lockdep_reader_access(x)
>> #endif
>>
>> +/**
>> + * RAW_SEQCNT_ZERO() - static initializer for raw_seqcount_t
>> + */
>> +#define RAW_SEQCNT_ZERO() 0
> 
> I am not sure why RAW_SWQCNT_ZERO() should be a function-like macro.
> 

I think I just went for consistency with SEQCNT_ZERO() -- but I agree,
that can just be simplified!

Thanks!


-- 
Thanks,

David / dhildenb





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