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Date: Wed, 02 Jan 2002 17:30:02 -0800 (PST)
From: John Baldwin <jhb@FreeBSD.org>
To: Bernd Walter <ticso@cicely9.cicely.de>
Subject: Re: When to use atomic_ functions? (was: 64 bit counters)
Cc: Peter Jeremy <peter.jeremy@alcatel.com.au>,
	Michal Mertl <mime@traveller.cz>, Bruce Evans <bde@zeta.org.au>,
	Mike Smith <msmith@FreeBSD.ORG>,
	Bernd Walter <ticso@cicely8.cicely.de>, arch@FreeBSD.ORG,
	Matthew Dillon <dillon@apollo.backplane.com>
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On 03-Jan-02 Bernd Walter wrote:
> On Wed, Jan 02, 2002 at 04:02:14PM -0800, Matthew Dillon wrote:
>> :Look at PCPU_GET/PCPU_SET.  Note that since an interrupt can preempt you
>> :and
>> :push you off onto another CPU, you have to use a critical section while
>> :updating per-CPU variables.  If desired, some kind of free area could be
>> :stuck
>> :in struct pcpu (or more likely, struct pcpu would hold a pointer to the
>> :area)
>> :that could be galloc/gfree'd or some such.
>> :
>> :-- 
>> :
>> :John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
>> :"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/
>> 
>>     Maybe we are going about this all wrong.  If a particular interface
>>     counter can only be modified from the device interrupt, or only be
>>     modified while holding the appropriate mutex, do we need any locking
>>     at all?
> 
> You need to hold the mutex while writing and reading.
> If you hold the mutex only while writing another CPU might still use
> old cached values.

Yes.

> The same goes for device interrrupt as it doesn't enshure inter CPU
> consistency.

Yes.

-- 

John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/

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