linux-kernel - Re: rwsem: down_read

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Date:	Wed, 05 May 2010 12:09:59 +0100
From:	David Howells <dhowells@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	dhowells@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Ying Han <yinghan@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: rwsem: down_read_unfair() proposal

Michel Lespinasse <walken@...gle.com> wrote:

> I only said it was doable :) Not done with the implementation yet, but I can
> describe the general idea if that helps. The high part of the rwsem is
> decremented by two for each thread holding or trying to acquire a write
> lock;

That would mean you're reducing the capacity of the upper counter by one since
the high part must remain negative if we're to be able to check it for
non-zeroness by checking the sign flag.  That means a maximum of 2^14-1 writers
queued on a 32-bit box (16384), but we can have more threads than that (up to
~32767).

Currently, we can have a maximum of 32767 writers+readers queued as we only
decrement the upper counter by 1 each time.

On a 64-bit box, the limitations go away for all practical purposes.

David
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