Question: I'm trying to create a button that hrefs to another page. Here's the HTML:

```
<div class="new_user"> 
<button id="buton_user_nou">Creaza User
</button>
</div>

```

and jQuery:

```
$('#buton_user_nou').click(function(){
location.href='/register.htm'
})

```

The button is completely inactive, in all browsers. It doesn't even highlight when hovering...
Any pointers please?

Answer: 
Short answer: no, it is incorrect to nest `kernel_fpu_begin()` calls, and it will lead to the userspace FPU state getting corrupted.

Medium answer: This won't work because `kernel_fpu_begin()` use the current thread's `struct task_struct` to save off the FPU state (`task_struct` has an architecture-dependent member `thread`, and on x86, `thread.fpu` holds the thread's FPU state), and doing a second `kernel_fpu_begin()` will overwrite the original saved state. Then doing `kernel_fpu_end()` will end up restoring the wrong FPU state.

Long answer: As you saw looking at the actual implementation in `<asm/i387.h>`, the details are a bit tricky. In older kernels (like the 3.2 source you looked at), the FPU handling is always "lazy" -- the kernel wants to avoid the overhead of reloading the FPU until it really needs it, because the thread might run and be scheduled out again without ever actually using the FPU or needing its FPU state. So `kernel_fpu_end()` just sets the TS flag, which causes the next access of the FPU to trap and cause the FPU state to be reloaded. The hope is that we don't actually use the FPU enough of the time for this to be cheaper overall.

However, if you look at newer kernels (3.7 or newer, I believe), you'll see that there is actually a second code path for all of this -- "eager" FPU. This is because newer CPUs have the "optimized" XSAVEOPT instruction, and newer userspace uses the FPU more often (for SSE in memcpy, etc). The cost of XSAVEOPT / XRSTOR is less and the chance of the lazy optimization actually avoiding an FPU reload is less too, so with a new kernel on a new CPU, `kernel_fpu_end()` just goes ahead and restores the FPU state. (

However in both the "lazy" and "eager" FPU modes, there is still only one slot in the `task_struct` to save the FPU state, so nesting `kernel_fpu_begin()` will end up corrupting userspace's FPU state.
Yes, as you defined some double variable & `foo` is also returning the double value; you have to use `kernel_fpu_begin` and `kernel_fpu_end` calls outside `foo` [also.](http://lists.kernelnewbies.org/pipermail/kernelnewbies/2012-November/006462.html)

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[Similar Problem](http://www.serverphorums.com/read.php?12,409657) also have this which have certain instances where you can code without using `kernel_fpu_begin` and `kernel_fpu_end` calls.