Date: Thu, 03 Jan 2002 12:58:39 +1100 From: Peter Jeremy <peter.jeremy@alcatel.com.au> To: Bernd Walter <ticso@cicely9.cicely.de> Cc: arch@FreeBSD.ORG Subject: Re: When to use atomic_ functions? (was: 64 bit counters) Message-ID: <20020103125839.H561@gsmx07.alcatel.com.au> In-Reply-To: <20020103011645.GE53199@cicely9.cicely.de>; from ticso@cicely9.cicely.de on Thu, Jan 03, 2002 at 02:16:45AM %2B0100 References: <200201012349.g01NnKA40071@apollo.backplane.com> <Pine.BSF.4.41.0201021003580.18429-100000@prg.traveller.cz> <20020103095701.B561@gsmx07.alcatel.com.au> <20020103002521.GB53199@cicely9.cicely.de> <20020103113919.E561@gsmx07.alcatel.com.au> <20020103011645.GE53199@cicely9.cicely.de>
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[Pruning CC list] On 2002-Jan-03 02:16:45 +0100, Bernd Walter <ticso@cicely9.cicely.de> wrote: >On Thu, Jan 03, 2002 at 11:39:20AM +1100, Peter Jeremy wrote: >> On 2002-Jan-03 01:25:22 +0100, Bernd Walter <ticso@cicely9.cicely.de> wrote: >> >My Alpha Architecture Handbook says that the barrier is unneeded. >> >I have no clue why they are there. >> >> You're right. Version 2, section 5.5 shows that they aren't needed >> when a single datum is being atomically updated (as needed here). >> They're only needed where the atomic operation is seizing a lock so >> that a larger structure can be atomically updated. > >I will do this change localy a send a patch to -alpha. >Maybe we can also remove the barriers for rel/acq in the non SMP case, >but I could also be wrong if drivers depend on them. My understanding is that barriers are needed when it's necessary to control the content of main memory or read/write ordering, as seen by something other than the current CPU. In a UP system, this means that barriers are still needed for I/O (both PIO and DMA). Based on atomic(9), I think the barriers are still necessary for acq/rel - which are essentially acquiring/releasing locks on larger data structures. Peter To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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