Date: Thu, 12 Jan 2006 19:52:37 +1100 From: Peter Jeremy <PeterJeremy@optushome.com.au> To: freebsd-hackers@freebsd.org Subject: Atomic operations across multiple processors Message-ID: <20060112085237.GA64401@cirb503493.alcatel.com.au>
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atomic(9) states: The current set of atomic operations do not necessarily guarantee atomic- ity across multiple processors. ... On the i386 architecture, the cache coherency model requires that the hardware perform this task, thus the atomic operations are atomic across multiple processors. On the ia64 architecture, coherency is only guaranteed for pages that are configured to using a caching policy of either uncached or write back. Unfortunately, this doesn't document the behaviour for other architectures - this makes it difficult to write portable code. For the ia64, the statement isn't especially helpful because there's no indication of what caching policy is used by default and how to change it. Also, it seems odd that write-back pages would be coherent whilst write-through pages aren't - is this a typo? The man page is also inconsistent with /sys/ia64/include/atomic.h which states that atomic operations _are_ SMP safe. I've tried looking at the mutex code to see how the iA64 achieves inter-processor synchronisation on top of (supposedly) non- synchronised atomic(9) primitives but can't find anything. I'd appreciate comments from people familiar with non-iA32 architectures. -- Peter Jeremy
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