Re: [PATCH -tip] introduce sys_membarrier(): process-wide memorybarrier (v9)

From: Paul E. McKenney
Date: Thu Mar 04 2010 - 11:50:37 EST

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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