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). > [ snip ] > >> +/** >> + * raw_seqcount_init() - runtime initializer for raw_seqcount_t >> + * @s: Pointer to the raw_seqcount_t instance >> + */ >> +# define raw_seqcount_init(s) __raw_seqcount_init(s) >> + >> #ifdef CONFIG_DEBUG_LOCK_ALLOC >> >> # define SEQCOUNT_DEP_MAP_INIT(lockname) \ >> @@ -111,11 +129,16 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s) >> # define seqcount_lockdep_reader_access(x) >> #endif >> >> +/** >> + * RAW_SEQCNT_ZERO() - static initializer for raw_seqcount_t >> + */ >> +#define RAW_SEQCNT_ZERO() 0 > > I am not sure why RAW_SWQCNT_ZERO() should be a function-like macro. > I think I just went for consistency with SEQCNT_ZERO() -- but I agree, that can just be simplified! Thanks! -- Thanks, David / dhildenb
