linux-kernel - Re: Why Semaphore Hardware-Dependent?

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Message-ID: <3450.1156877812@warthog.cambridge.redhat.com>
Date:	Tue, 29 Aug 2006 19:56:52 +0100
From:	David Howells <dhowells@...hat.com>
To:	Andi Kleen <ak@...e.de>
Cc:	David Howells <dhowells@...hat.com>,
	Christoph Lameter <clameter@....com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Dong Feng <middle.fengdong@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Why Semaphore Hardware-Dependent? 

Andi Kleen <ak@...e.de> wrote:

> > > BTW maybe it would be a good idea to switch the wait list to a hlist,
> > > then the last user in the queue wouldn't need to 
> > > touch the cache line of the head. Or maybe even a single linked
> > > list then some more cache bounces might be avoidable.
> > 
> > You need a list_head to get O(1) push at one end and O(1) pop at the other.
> 
> The poper should know its node address already because it's on its own stack.

No.  The popper (__rwsem_do_wake) runs in the context of up_xxxx(), not
down_xxxx().  Remember: up() may need to wake up several processes if there's
a batch of readers at the front of the queue.

Remember also: rwsems, unlike mutexes, are completely fair.

> > In addition a singly-linked list makes interruptible ops non-O(1) also.
> 
> When they are interrupted I guess? Hardly a problem to make that slower.

Currently interruptible rwsems are not available, but that may change, and
whilst I agree making it slower probably isn't a problem, it's still a point
that has to be considered.

David
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