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

[David Hildenbrand <david@redhat.com>]

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@redhat.com> 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,

David / dhildenb