On Dec 17, 2021, at 3:30 AM, David Hildenbrand david@redhat.com 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?