On 17.12.21 18:29, David Hildenbrand wrote:

On 17.12.21 18:02, Nadav Amit wrote:

quoted

quoted

On Dec 17, 2021, at 3:30 AM, David Hildenbrand [off-list ref] 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