linux-kernel - Re: [patch] generic rwsems

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Date:	Fri, 13 Apr 2007 12:25:18 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>, Andi Kleen <ak@...e.de>,
	David Howells <dhowells@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch] generic rwsems

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.
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