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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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