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

[Nadav Amit]

> On Dec 17, 2021, at 9:49 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
> 
> On 17.12.21 18:29, David Hildenbrand wrote:
>> 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).
> 
> Sorry, I forgot to mention an important point: the raw_seqcount_t
> doesn't give you any additional "power" to abuse.
> 
> You can just use the ordinary seqcount_t with the raw_ functions. One
> example is mm->write_protect_seq . So whatever we would want to "invent"
> should also apply to the raw_ functions in general --  which might be
> undesired or impossible (IIRC IRQ context).
> 

Thanks for the clarification. I was unfamiliar with
raw_read_seqcount_begin() (and friends). Indeed it is very very rarely
used.