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Date: Thu, 9 Aug 2007 21:19:20 +0200 From: Segher Boessenkool <segher@...nel.crashing.org> To: Chris Snook <csnook@...hat.com> Cc: wjiang@...ilience.com, schwidefsky@...ibm.com, wensong@...ux-vs.org, heiko.carstens@...ibm.com, linux-kernel@...r.kernel.org, ak@...e.de, cfriesen@...tel.com, netdev@...r.kernel.org, paulmck@...ux.vnet.ibm.com, horms@...ge.net.au, akpm@...ux-foundation.org, linux-arch@...r.kernel.org, jesper.juhl@...il.com, davem@...emloft.net, torvalds@...ux-foundation.org, zlynx@....org, rpjday@...dspring.com Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha >>>> The only safe way to get atomic accesses is to write >>>> assembler code. Are there any downsides to that? I don't >>>> see any. >>> >>> The assumption that aligned word reads and writes are atomic, and >>> that words are aligned unless explicitly packed otherwise, is >>> endemic in the kernel. No sane compiler violates this assumption. >>> It's true that we're not portable to insane compilers after this >>> patch, but we never were in the first place. >> You didn't answer my question: are there any downsides to using >> explicit coded-in-assembler accesses for atomic accesses? You >> can handwave all you want that it should "just work" with >> volatile accesses, but volatility != atomicity, volatile in C >> is really badly defined, GCC never officially gave stronger >> guarantees, and we have a bugzilla full of PRs to show what a >> minefield it is. >> So, why not use the well-defined alternative? > > Because we don't need to, You don't need to use volatile objects, or accesses through valatile-cast pointers, either. > and it hurts performance. Please show how it does this -- one load is one load either way, and one store is one store. Segher - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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