Re: Atomic operations (SPARC)

From: Paul H. Hargrove (PHHargrove_at_lbl_dot_gov)
Date: Mon Aug 24 2009 - 12:09:20 PDT

  • Next message: Alan Woodland: "Re: Atomic operations (SPARC)"
    Alan Woodland wrote:
    > Hello,
    > I just noticed building on SPARC is failing. Log of a failed build is:
    >  Debian builds for 'sparclite' apparently in a lowest common
    > denominator strategy.
    > This comes from the membar instruction in several functions in
    > libcr/arch/sparc/cr_atomic.h right? It looks like there's half-written
    > support for avoiding membar.
    > Would it be feasible (and sensible?) for me to put together a patch
    > that implements some/all of cr_atomic.h using these gcc builtins?
    > Where hardware support exists they are translated into the appropriate
    > instruction, where it doesn't they become a function call.
    > Is there any reason not to use these builtins for all arches and avoid
    > having to maintain a cr_atomic.h for every supported platform?
    > Alan
    There are at least 4 parts to the answer.
    0) I'd be surprised if the SPARC architecture it usable, even if it 
    compiles.  It is documented to be incomplete and to support (IIRC) only 
    UltraSPARC and higher (see #2, below).
    1) The gcc atomic builtins were not (widely?) available when we started 
    this project, and until recently we supported kernels as old as 2.4.0, 
    which required an ancient gcc-2.96.  If you were to implement an 
    arch/generic in the same spirit as the kernel's asm-generic I think we 
    could use it when support exists in the compiler, and as a porting aid, 
    but we'd probably not throw away the existing implementations.
    2) Sparclite does not have the required atomic instructions.  With only 
    a test-and-set or load-and-clear type of atomic support one cannot 
    implement a signal-safe compare-and-swap in user space (kernel does it 
    by blocking interrupts momentarily).  This is true of "older" ARM as 
    well, but on that platform there is a neat kernel trick implemented 
    by/for the NPTL port using the equivalent of a VDSO to use the atomic 
    instructions on "newer" ARM or use the kernel for help otherwise.  I 
    don't know of such a trick on sparclite, but NPTL must work somehow.
    3) Ever since we dropped 2.4.x kernel support, I have been wondering if 
    we could drop our user-space atomics entirely in favor of using 
    futexes.  However, I've not had the opportunity to examine that possibility.
    Paul H. Hargrove                          PHHargrove_at_lbl_dot_gov
    Future Technologies Group                 Tel: +1-510-495-2352
    HPC Research Department                   Fax: +1-510-486-6900
    Lawrence Berkeley National Laboratory     

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