Re: IBM test question

[Matthieu CASTET <matthieu.castet@parrot.com>]

Hi,

Esben Nielsen wrote:
> 
>>>> for userland atomic operations (aside from swapping).
>>> I can, if I use a kernel helper :) [1]
>>
>>  Yep, but much slower.
>>
> 
> I worked with an ARMv4 at my former job and wanted to run Linux on it. I 
> thus gave this problem a thought. I got the following idea:
> Make a user space preemt-disable counter just like the in-kernel one. 
> This can be done by registering a address in userspace per thread 
> pointing where to find the counter. When the kernel wants to schedule it 
> checks if the counter is non-zero. If it is (the very rare case), it 
> doesn't reschedule but sets up a timer of some configurable time (say 1 
> ms or
> whatever you need). If the counter is not back to 0 after the timer has 
> expired we schedule anyway and signals the thread to let it know that an
> atomic operation have failed. Notice, that this can only happen due to an
> error in the program: You must always be able finish your atomic 
> operations in 1 ms.
> 
> (There are a lot of details to this, ofcourse.  Forinstance. in the case 
> the kernel wanted to schedule and sets up he timer, the user space 
> program needs to know it so it can disable the timer reschedule as soon 
> as the counter reaches 0. And there is the problem of not swapping out 
> the page where the counter is stored....)
> 

The kernel helper is not slow for armv5. There no userspace->kernel 
switch with some magic. That's just 15 instructions instead of one.
The kernel helper is at a special address. When a context switch occurs, 
  the kernel check if it wasn't in the helper and finish the atomic 
operation or set a flag.


Matthieu