Re: [patch] generic rwsems

[Eric Dumazet]

On Fri, 13 Apr 2007 14:31:52 +0100
David Howells <dhowells@xxxxxxxxxx> 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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