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From: David Howells <dhowells@redhat.com>
In-Reply-To: <20100505032033.GA19232@google.com>
References: <20100505032033.GA19232@google.com>
To: Michel Lespinasse <walken@google.com>
Cc: dhowells@redhat.com, Andrew Morton <akpm@google.com>,
       Linux-MM <linux-mm@kvack.org>, Ying Han <yinghan@google.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: rwsem: down_read_unfair() proposal
Date: Wed, 05 May 2010 11:06:44 +0100
Message-ID: <22994.1273054004@redhat.com>
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Michel Lespinasse <walken@google.com> wrote:

> and looks like it's doable with the x86 rwsem implementation as well in a
> way that would only involve changes to the rwsem spinlock-protected slow
> paths in lib/rwsem.c .

It's not as easy as it seems.  Once an XADD-based rwsem is contended, you
cannot necessarily tell without looking at the queue whether the rwsem is
currently write-locked or read-locked.

David