Date: Thu, 15 Dec 2005 23:46:26 -0800
From: Jeremy Higdon <jeremy@sgi.com>
To: Keith Owens <kaos@sgi.com>
Cc: paulmck@us.ibm.com, Andi Kleen <ak@suse.de>,
       ajwade@cpe001346162bf9-cm0011ae8cd564.cpe.net.cable.rogers.com,
       vatsa@in.ibm.com, Oleg Nesterov <oleg@tv-sign.ru>,
       linux-kernel@vger.kernel.org, Dipankar Sarma <dipankar@in.ibm.com>,
       Andrew Morton <akpm@osdl.org>, Ingo Molnar <mingo@elte.hu>
Subject: Re: Semantics of smp_mb() [was : Re: [PATCH] Fix RCU race in access of nohz_cpu_mask ]
Message-ID: <20051216074626.GB201289@sgi.com>
References: <20051213162027.GA14158@us.ibm.com> <17158.1134512861@ocs3.ocs.com.au>
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Roland Dreier got this right.  The purpose of the mmiowb is
to ensure that writes to I/O devices while holding a spinlock
are ordered with respect to writes issued after the original
processor releases and a second processor acquires said
spinlock.

A MMIO read would be sufficient, but is much heavier weight.

On the SGI MIPS-based systems, the "sync" instruction was used.
On the Altix systems, a register on the hub chip is read.

>From comments by jejb, we're looking at modifying the mmiowb
API by adding an argument which would be a register to read
from if the architecture in question needs ordering in this
way but does not have a lighter weight mechanism like the Altix
mmiowb.  Since there will now need to be a width indication,
mmiowb will be replaced with mmiowb[bwlq].

jeremy
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