> 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.
