Messages in this thread | | | Date | Thu, 4 Mar 2010 08:50:20 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH -tip] introduce sys_membarrier(): process-wide memory barrier (v9) |
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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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