linux-kernel - Re: [patch] generic rwsems

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Date:	Fri, 13 Apr 2007 16:29:09 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch] generic rwsems

On Fri, 13 Apr 2007 14:31:52 +0100
David Howells <dhowells@...hat.com> wrote:

> Break the counter down like this:
> 
> 	0x00000000	- not locked; queue empty
> 	0x40000000	- locked by writer; queue empty
> 	0xc0000000	- locket by writer; queue occupied
> 	0x0nnnnnnn	- n readers; queue empty
> 	0x8nnnnnnn	- n readers; queue occupied

If space considerations are that important, we could then reserve one bit for the 'wait_lock spinlock'

0x20000000 : one cpu gained control of 'wait_list'

This would save 4 bytes on 32 bit platforms.

64 bit platforms could have a limit of 2^60 threads, instead of the way too small 2^28 one ;)

(we loose the debug version of spinlock of course)

Another possibility to save space would be to move wait_lock/wait_list outside of rw_semaphore, in a hashed global array.
This would save 12/16 bytes per rw_semaphore (inode structs are probably the most demanding)
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