From owner-freebsd-current  Sat Jul 10 14:41:29 1999
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Date: Sat, 10 Jul 1999 14:41:26 -0700 (PDT)
From: Matthew Dillon <dillon@apollo.backplane.com>
Message-Id: <199907102141.OAA58697@apollo.backplane.com>
To: Alan Cox <alc@cs.rice.edu>,
	Stephen McKay <syssgm@detir.qld.gov.au>,
	Andrew Gallatin <gallatin@cs.duke.edu>, freebsd-current@FreeBSD.ORG
Subject: "objtrm" problem probably found (was Re: Stuck in "objtrm")
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    The supposedly atomic functions in i386/include/atomic.h are not
    as atomic as was previously thought :-):

#define atomic_add_short(P, V)          (*(u_short*)(P) += (V))

    I looked at that kinda funny.  But C doesn't guarentee a RMW opcode
    for a "+=" !!!.   Alan found an example showing a load, add, and
    separate store by disassembling the kernel.  An interrupt occuring in
    the middle of that sequence would lead to the reported problems.

    I'm pretty sure this is the problem causing the "objtrm" lockups.  I've
    submitted a patch to Alan to make those atomic_*() functions inline
    assembly.

    There also appear to be a number of mistakes in some of the inline
    assembly already laying around -- some of it appears to specify output
    arguments for RMW instructions without also indicating to the compiler
    that those are input arguments as well, leading to bogus optimizations 
    by the C compiler.  The 'setbits' function is the one Alan found
    inadvertantly while we were trying to figure out why my first iteration
    didn't work.  heh heh.  Alan will fix the setbits inline.

    It wouldn't hurt if someone with more assembly experience could audit 
    all of the inline assembly present in the source base.

    I am re-running the 16MB/make -j5 buildworld test to see if this has
    really fixed the problem.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>



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