Date: Wed, 29 Mar 2000 21:44:22 -0800 From: Alfred Perlstein <bright@wintelcom.net> To: Allen Pulsifer <pulsifer@mediaone.net> Cc: Mike Smith <msmith@FreeBSD.ORG>, Matthew Dillon <dillon@apollo.backplane.com>, freebsd-current@FreeBSD.ORG Subject: Re: Using packed structs to gain cheap SMP primatives Message-ID: <20000329214422.X21029@fw.wintelcom.net> In-Reply-To: <NBBBJNDFEKPEHPFCLNLHMEIJGHAA.pulsifer@mediaone.net>; from pulsifer@mediaone.net on Wed, Mar 29, 2000 at 11:36:05PM -0500 References: <20000329192526.U21029@fw.wintelcom.net> <NBBBJNDFEKPEHPFCLNLHMEIJGHAA.pulsifer@mediaone.net>
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* Allen Pulsifer <pulsifer@mediaone.net> [000329 21:05] wrote:
> Here's another alternative for reading structures like time
> that always change monotonically: read the values in
> "MSB" to "LSB" order, then go back and check in reverse
> order that nothing has changed. For example, to read a
> structure containing hours, minutes, seconds:
>
> for (;;)
> { h = timep->hour;
> m = timep->minute;
> s = timep->second;
> if (m != timep->minute) continue;
> if (h != timep->hour) continue;
> break;
> }
>
> The assumption is that from the instant you first read
> timep->hour until the instant you double check its value,
> it could not have wrapped all the way back around to its
> previous value. Or to put it another way, if it has
> succeeding in wrapping all the way around, having a
> correct snapshot of the structure is the least of your
> problems and the value you use is arbitary.
>
> This same method can be used to read the MSW and LSW of
> any counter-like structure that is updated by an interrupt.
>
> Note this method will not work for a structure that can
> both increment and decrement--it has to be only one or
> the other.
I'm aware of this, the problem is that tz may move in either
direction. Hence my question about using sizes that are machine
atomic for read/write. :)
I've got several books on various systems here and I don't remeber
any of them mentioning a problem with 32bit aligned updates being
atomic.
-Alfred
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