Re: [PATCH -tip] introduce sys_membarrier(): process-wide memorybarrier (v9)
From: Linus Torvalds
Date: Sat Mar 06 2010 - 14:45:25 EST
On Thu, 4 Mar 2010, Ingo Molnar wrote:
> Perhaps NOFPU could do lazy context saving: clear the TS flag and only save
> the FPU state if it's actually used by the signal handler?
If we can get that working reliably, we probably shouldn't use NOFPU at
all, and we should just do it unconditionally. That big (and almost always
pointless) FPU state save is a _big_ performance issue on signal handling,
and if we can do it lazily, we should.
However, I'm not at all convinced we can do this reliably. How do we
detect the "signal frame is dead" case with things like siglongjmp() etc?
And if we can't detect that "frame no longer exists", we can't really do
the lazy context saving.
Now, there's _also_ the issue of the signal handler function possibly
actually looking at the FPU state on the stack, and for that, a SA_NOFPU
would be a good way to say "you can't do that". So it's possible that even
if we could reliably detect the frame liveness we'd really have to use
that new flag anyway.
But if we do need a SA_NOFPU flag, then that means that basically no app
will use it, and it will be some special case for some really unusual
library. So I really don't think this whole thing is worth it unless you
could do it automatically.
(The "user accesses the frame" case _could_ possibly be handled by
pointing the FP frame to a special faulting location, and never nesting
the FP optimization. Nested signal handlers are unusual enough that they
aren't worth optimizing for anyway. So I'm sure that there are possible
solutions for "automatically just do the right thing" in theory, but I
suspect they get rather complex)
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