unseen/unknown function definition in linux source

Question

Can any one please explain this piece of code I found in the linux kernel source. I see a lots of code like this in linux and minix kernel but dont seem to find what it does (even if C compilers support that kind of function definition)

/* IRQs are disabled and uidhash_lock is held upon function entry.
 * IRQ state (as stored in flags) is restored and uidhash_lock released
 * upon function exit.
 */
static void free_user(struct user_struct *up, unsigned long flags)
__releases(&uidhash_lock)
{
uid_hash_remove(up);
spin_unlock_irqrestore(&uidhash_lock, flags);
key_put(up->uid_keyring);
key_put(up->session_keyring);
kmem_cache_free(uid_cachep, up);
}

I cannot find out if this reference __releases(&uidhash_lock) before the parenthesis starts is allowed OR supported. (It sure is supported as it is in the linux kernel)

Answer 1

These are annotations used by Sparse, the Static Analysis Tool for the Linux Kernel, originally written by Linus Torvalds. When compiled normally, without Sparse, they are simply #defined away to nothing.

The definitions are in include/linux/compiler.h:

# define __releases(x)  __attribute__((context(x,1,0)))

This one uses the __attribute__ non-standard GCC extension.

This specific annotation is paired with another one called __acquires(x), which allows Sparse to determine whether the code paths which acquire and release a specific lock are correctly balanced.