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

[Nadav Amit]

> 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?

[ 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.

Moreover, the documentation showed:

+	/* static */
+	static raw_seqcount_t foo_seqcount = RAW_SEQCNT_ZERO(foo_seqcount);
+

But RAW_SEQCNT_ZERO does not have an argument?