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, David / dhildenb
