Date: Mon, 14 Apr 2008 08:56:24 -0700 From: Marcel Moolenaar <xcllnt@mac.com> To: Jeff Roberson <jroberson@jroberson.net> Cc: arch@freebsd.org, Arthur Hartwig <Arthur.Hartwig@nokia.com> Subject: Re: f_offset Message-ID: <300DE361-167E-4491-8E8C-7A227225B506@mac.com> In-Reply-To: <20080413222626.X959@desktop> References: <20080412132457.W43186@desktop> <480313A2.4050306@nokia.com> <20080413222626.X959@desktop>
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On Apr 14, 2008, at 1:27 AM, 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? 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. I'm working on SMP for PowerPC.. -- Marcel Moolenaar xcllnt@mac.com
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