Re: [patch] generic rwsems

[Nick Piggin <npiggin@suse.de>]

On Fri, Apr 13, 2007 at 12:04:16PM +0200, Nick Piggin wrote:
> OK, this patch is against 2.6.21-rc6 + Mathieu's atomic_long patches.
> 
> Last time this came up I was asked to get some numbers, so here are
> some in the changelog, captured with a simple kernel module tester.
> I got motivated again because of the MySQL/glibc/mmap_sem issue.
> 
> This patch converts all architectures to a generic rwsem implementation,
> which will compile down to the same code for i386, or powerpc, for
> example, and will allow some (eg. x86-64) to move away from spinlock
> based rwsems.

Oh, and it also converts powerpc and sparc64 to 64-bit counters, so
they can handle more than 32K tasks waiting (which was apparently a
real problem for SGI, and is probably a good thing).

But that reminds me:
> +/*
> + * the semaphore definition
> + */
> +struct rw_semaphore {
> +	atomic_long_t		count;
> +	spinlock_t		wait_lock;
> +	struct list_head	wait_list;
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +	struct lockdep_map dep_map;
> +#endif
> +};

I think I should put wait_lock after wait_list, so as to get a better
packing on most 64-bit architectures.