Date: Mon, 14 Apr 2008 18:49:33 +1000 From: Arthur Hartwig <Arthur.Hartwig@nokia.com> To: ext Jeff Roberson <jroberson@jroberson.net> Cc: arch@freebsd.org Subject: Re: f_offset Message-ID: <48031A9D.3050806@nokia.com> In-Reply-To: <20080413222626.X959@desktop> References: <20080412132457.W43186@desktop> <480313A2.4050306@nokia.com> <20080413222626.X959@desktop>
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ext Jeff Roberson wrote: > > On Mon, 14 Apr 2008, Arthur Hartwig wrote: > >> ext Jeff Roberson wrote: >>> So I'm in the midst of working on other filesystem concurrency >>> issues and that has brought me back around to f_offset again. I'm >>> working on a method to allow non-overlapping writes and reads to >>> proceed concurrently to the same file. This means the exclusive >>> vnode lock can not be used to protect f_offset even in the write case. >>> >>> To maintain the existing semantics I'm simply going to add an >>> exclusive sx_xlock() around access to f_offset. This is done >>> inconsistently today which is fine from the perspective of the >>> updates in most cases being user-space races. However, f_offset is >>> 64bit and can not be written atomically on 32bit systems and so >>> requires some extra synchronization there. >> I'm not sure of the processor family constraints of the i386 builds, >> but the Intel IA32 architecture manual says reads and writes of a >> quadword (64 bits) aligned on a quadword boundary are atomic (Pentium >> and newer CPUs). Guess that leaves out i386, i486 (any others?) > > Thanks. I hadn't seen that. Do you know which manual and section > states this? Intel 64 and IA-32 Architectures Software Developer's Manual Vol 3A: System Programming Guide, Part 1, section 7.1.1 Guaranteed Atomic Operations. You can download this (and other volumes of the Intel Architecture manuals) from http://www.intel.com/products/processor/manuals/index.htm > I was intending to simply use cmpxchg8b but it sounds like that may > not be necessary. We still have to handle other 32bit archs like > powerpc and mips but I'm not sure if any of those are SMP. Do you also have to handle i386 and i486? > > Jeff
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