Re: [patch] generic rwsems

Eric Dumazet
2007-04-13T14:29:09Z

On Fri, 13 Apr 2007 14:31:52 +0100
David Howells <dhowells@redhat.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)