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    SubjectRe: [PATCH -tip] introduce sys_membarrier(): process-wide memory barrier (v9)
    On Thu, Mar 04, 2010 at 08:34:16AM -0800, Linus Torvalds wrote:
    > On Thu, 4 Mar 2010, Ingo Molnar wrote:
    > >
    > > - SA_NOFPU: on x86 to skip the FPU/SSE save/restore, for such fast in/out special
    > > purpose signal handlers? (can whip up a quick patch for you if you want)
    > I'd love to do this, but it's wrong.
    > It's too damn easy to use the FPU by mistake in user land, without ever
    > being aware of it. memset()/memcpy are obvious potential users SSE, but
    > they might be called in non-obvious ways implicitly by the compiler (ie
    > structure copy and setup).
    > And modern glibc ends up using SSE4 even for things like strstr and
    > strlen, so it really is creeping into all kinds of trivial helper
    > functions that might not be obvious. So SA_NOFPU is a lovely idea, but
    > it's also an idea that sucks rotten eggs in practice, with quite possibly
    > the same _binary_ working or not working depending on what kind of CPU and
    > what shared library it happens to be using.
    > Too damn fragile, in other words.
    > (Now, if it's accompanied by the kernel actually _testing_ that there is
    > no FPU activity, by setting the TS flag and checking at fault time and
    > causing a SIGFPE, then that would be better. At least you'd get a nice
    > clear signal rather than random FPU state corruption. But you're still in
    > the situation that now the binary might work on some machines and setups,
    > and not on others.

    I was assuming that using the FPE in the special handler would result in
    a SIGFPE -- but that it would not affect normal signal handlers, only
    those invoked by this user-level-RCU acceleration mechanism.

    Thanx, Paul

    > > - SA_RUNNING: a way to signal only running threads - as a way for user-space
    > > based concurrency control mechanisms to deschedule running threads (or, like
    > > in your case, to implement barrier / garbage collection schemes).
    > Hmm. This sounds less fundamentally broken, but at the same time also
    > _way_ more invasive in the signal handling layer. It's already one of our
    > more "exciting" layers out there.
    > Linus

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